Changes in mass and dimensions of sunflower ( Helianthus annuus L.) Achenes and seeds due to...

Macrobotanical remains of fruits and seedsfound in the archaeological record are essential toour understanding of human cultural develop-ments. Carbonization is one of the processes bywhich these remains survive the physical andchemical decomposition associated with burial atan archaeological site. In this context, carboniza-tion can be described as the exposure of fruits andseeds to any heat source in the absence of air. Sub-sequently, the heat or thermal energy is thentransferred by radiation from the heat source tothe plant tissues (Braadbaart et al. 2004a).

While carbonization can facilitate the preserva-

tion of plant remains, it also can cause variousdistortions in the physical properties of the fruitsand seeds which may impact their shapes and di-mensions (Hopf 1955; van Zeist 1970; Kislevand Rosenzweig 1989; Smith and Jones 1990;Wright 1998, 2003). Because archaeobotanistsoften rely on lengths, widths, and diameters ofthe carbonized fruits and seeds to determine if aplant remain reflects a wild or a domesticatedspecies, the distortions can impede their ability tomeasure and to reconstruct the precarbonized di-mensions necessary for assessing domestication.

In North America, it is common to differenti-ate between wild and domesticated forms of sun-flower (Helianthus annuus L.) based on the mea-sured length and width or a calculated size index(Lentz et al. 2001). For sunflower, an L*W index

Changes in Mass and Dimensions of Sunflower (Helianthusannuus L.) Achenes and Seeds Due to Carbonization1

Freek Braadbaart2 and Patti J. Wright3,*

2Department of Earth Sciences–Geochemistry, Faculty of Geosciences, Utrecht University, P.O. Box80021, 3058TA Utrecht, the Netherlands; e-mail: [email protected]

3Anthropology Department, University of Missouri–St. Louis, St. Louis, MO 63121, USA; e-mail:[email protected]

*Corresponding author

Changes in Mass and Dimensions of Sunflower (Helianthus annuus L.) Achenes and SeedsDue to Carbonization. When analyzing sunflower (Helianthus annuus L.) remains, whichare often carbonized, archaeobotanists commonly differentiate between wild and domesti-cated achenes and seeds based on the measured length (L) and width (W) or the calculatedindex L*W. Carbonization reduces the dimensions. To compensate for these reductions, ar-chaeobotanists use a single correction factor proposed by Richard Yarnell (1978) for allcases. The use of a single correction factor can bias the reconstructed dimensions as car-bonization is a highly variable process. The current study determines the relationship be-tween carbonization and the dimensions of length and width. Measurements establishedthat a decrease of 2.5–22.5% in achene length and 10–29% in achene width can occur, de-pending on temperature, heating rate, and variety. For seeds, temperature is of most impor-tance, and shrinkage ranges from 0–27% for the length and from 0–20% for the width.These ranges make the use of a single correction factor problematic. A method is developedin which reflectance (an optical property applied in coal technology to determine coal rank)is used to measure the carbonization temperature, and in turn the shrinkage can be calcu-lated. Subsequently, correction factors are calculated to reconstruct the original length andwidth. When applied to an assemblage of carbonized sunflower achenes, the newly devel-oped method shows that the Yarnell single correction factor may bias the dimensions to-wards classifications of “wild” or “ruderal” forms of sunflower.

Key Words: Sunflower achenes and seeds, Helianthus annuus, carbonization, reflectance,paleoethnobotany.

Economic Botany, 61(2), 2007, pp. 137–153.© 2007, by The New York Botanical Garden Press, Bronx, NY 10458-5126 U.S.A.

1 Received 25 March 2006; accepted 30 January2007.

is used in conjunction with a single correction fac-tor (accounting for the shrinkage associated withcarbonization) to estimate the original size of theachene or seed (Yarnell 1978). However, physicalas well as chemical properties depend strongly onthe variables that determine the carbonizationprocess (Braadbaart and Bakels n.d.), and it canbe expected that changes in the dimensions of theachenes and seeds will depend on these variables.In this study, two types of variables are recog-nized: (1) more general variables related to theheat source, and (2) more specific variables relatedto the samples under investigation.

Studies on various fruits and seeds show thattheir physical properties and chemical structurechange as a function of the temperature and thetime of exposure to the heat source (Braadbaart2004b). The primary process that results fromcarbonization is the gradual conversion of themain constituents of the plant tissues (polysaccha-rides, protein, lipids, and lignin) into a range ofaromatic moieties. Simultaneously, many volatilesare formed that cause a considerable mass loss anda corresponding shrinkage of the original material.

Braadbaart et al. (2007) assess the influenceof carbonization on the chemical structure of asingle variety (Arikara) of sunflower achenesand seeds. Results indicate that at 310°C theoriginal constituents (polysaccharides, pro-teins, and lignin) are converted into aromaticcompounds, and most lipids evaporate ratherthan carbonize. Between 370 and 400°C, allthe lipids have disappeared from the residues.

The current research expands upon the 2007article. We have broadened our study to includetwo other varieties of sunflowers and focused thisresearch on the physical changes in dimensionthat are critical to the evaluation of archaeologicalsunflower remains. In sum, we found that impactsof carbonization on mass and dimensions are afunction of heat source and sample-related vari-ables. In this article, we detail information aboutthose interrelationships and present a method thatcan not only enhance our understanding of ther-mal exposure, but also can, with more precision,aid in correcting for carbonization.

Samples and MethodsSamples

Modern sunflowers (Helianthus annuus L. var.Arikara) were grown by Wright in an experimen-tal garden situated in St. Louis, Missouri, U.S.A.

138 ECONOMIC BOTANY [VOL. 61

Seeds were obtained from the North Central Re-gional Plant Introduction Station of the UnitedStates National Germplasm System. Initially, thevariety, Arikara, was chosen based on conversa-tions with Charles Heiser (personal communica-tion 2004; Heiser 1953), who, along with Rich-ard Yarnell, conducted seminal research inunderstanding the carbonization and domestica-tion of the sunflower in North America. TheArikara achenes were harvested in the fall of2004. To better assess the range of effects that oc-curs with carbonization, we expanded our experi-ments to include other varieties of sunflowers. Inkeeping with previous experiments conducted byWright (2003), we chose the variety Hopi BlackDye. We also selected a third variety, ApacheBrown. These latter two varieties were obtainedin bulk from Native Seed SEARCH, a privatelyoperated U.S. seed bank dedicated to the preser-vation to the preservation of Native Americanstrains of crops. Combined, the three varietiesrepresent a diverse range of sizes and shapes.

Because achenes as well as isolated seeds are re-covered from archaeological contexts, we experi-mented with both forms. The seeds for the labo-ratory experiments were separated from achenesby manually removing the pericarp or hull.Lengths and widths of the total population(n = 100) were measured (Fig. 1), and masseswere obtained of each variety. These data aresummarized in Table 1. In order to reduce the

Fig. 1. Key to the measured dimensional featuresand the orientation of sunflower achenes on the scan-ner bed. For seeds the same procedure is followed.

size of the samples to be investigated, 95% confi-dence intervals were calculated (Table 1). The re-sults showed that for sunflower achenes andseeds, random samples of n = 10 are sufficient tofit the 95% confidence interval for length andwidth. Means of lengths and widths for eachsample were used to investigate the influence ofcarbonization on changes in dimensions.

To illustrate the potentials of our method for re-constructing dimensions, we applied our methodof reflectance on archaeological achenes. These ach-enes were derived from the Stelzer Site (23SC910),St. Charles County, Missouri (Harl and Wright1994). These samples are referred to as “archaeolog-ical achenes or seeds.”

ReflectanceThe carbonization process and its governing

variables, as well as the application of reflectance

2007] BRAADBAART AND WRIGHT: CARBONIZED SUNFLOWER SEEDS 139

measurements, have been discussed extensivelyelsewhere (Braadbaart 2004b). Because they playan important role in this study, these phenomenawill be briefly described. In particular, reflectancemeasured on carbonized organic material, in thiscase sunflower achenes and seeds, can be used todetermine the temperature at which they havebeen carbonized. The main constituents of ach-enes are cellulose, lignin, proteins, and oil. Withincreasing temperatures, the oils evaporate, andthe chemical structure of the other three con-stituents gradually changes into aromatic ringstructures. These aromatic rings are more planarthan their precursors and tend to align with eachother, causing more light to reflect. Reflectance ismeasured through the optical properties, i.e., therefractive and absorption indices of the aromaticrings, and is defined as the percentage of verti-cally incident monochromatic light reflected

Table 1. a. Length and width of the investigated sunflower achenes.

Arikara Hopi Black Dye Apache Brown

Length (mm) Mean (n = 100) 11.95 10.30 11.51S.D. 1.04 0.74 0.67Degree of variation 9.40–14.10 8.55–12.04 9.80–13.17

Width (mm) Mean (n = 100) 6.53 4.85 8.30S.D. 0.95 0.81 0.81Degree of variation 4.38–9.40 3.84–9.24 5.91–10.48

95% conf. interval length (mm) 11.31–12.60 9.57–10.57 11.13–11.97(n = 10) width (mm) 5.94–7.12 6.18–7.2 7.94–8.94

b. Length and width of the investigated sunflower seeds.


Length (mm) Mean (n = 100) 8.38 7.68 8.37S.D. 1.17 0.62 0.66Degree of variation 5.69–10.37 6.36–9.74 7.20–10.02

Width (mm) Mean (n = 100) 4.18 4.16 4.73S.D. 0.56 0.65 0.56Degree of variation 2.96–5.43 2.84–5.86 3.60–6.11

95% conf. interval length (mm) 7.66–9.10 7.30–8.06 8.06–8.88(n = 10) width (mm) 3.82–4.55 3.76–4.56 4.38–5.08

c. Weight of investigated achenes and seeds.


Mean mass of Achene 0.7780 (100%) 0.8850 (100%) 1.2051 (100%)samples of 10 Seed 0.4231 (53.7%) 0.3675 (41.5%) 0.4947 (41.1%)specimens (gr) Hull 0.3649 (46.3%) 0.5175 (58.5%) 0.7104 (58.9%)

from a highly polished surface of a sample cali-brated against the light reflected from a standardof known reflectance. At temperatures of approxi-mately 270°C and higher, the formation ofmulti-ring (3–5) aromatic structural units causesan increase in reflectance (Carr and Williamson1989; Braadbaart 2004b). From about 440°C,condensation and further molecular reorganiza-tion associated with the formation of large pol-yaromatic sheets further increases the reflectance.The dependence on chemical structure, whichdetermines the optical properties, means that re-flectance can be used as an indirect method forresolving differences in the chemical structure.

To determine reflectance (%Rr), the followingprocedures were followed. From each sample car-bonized at a given temperature, three specimenswere embedded in resin blocks and polished. Foreach specimen, 100 randomly performed re-flectance readings were performed in order to ob-tain a statistically acceptable population. Fromthat population, the mean reflectance of eachspecimen was calculated. Reflectance was mea-sured under oil immersion at a wavelength of 546nm using a Leitz motorized DMLA microscopeequipped with a xyz-stage and a Basler videocamera (Veld 2006). Preparation of polishedblocks and reflectance measurements were per-formed according to standard methods defined inISO 7404, Part 2 (1985), and ISO 7404, Part 5(1994). The same set of procedures was followedto measure the reflectance of the archaeologicalspecimens. The results were compared to the ex-perimental samples to determine the carboniza-tion regime and, in turn, to estimate the shrink-age of the archaeological specimens.

Digital Imaging AnalysisDigital imaging analysis was used to record di-

mensions (Braadbaart and van Bergen 2005).This technique offers an objective and quantita-tive method to extract digital images from fruitsand seeds and their carbonized residues. Theseimages can be used to measure dimensions of in-dividual specimens and mathematically extractsize and shape-related information. For the exper-iments, length and width of achenes and seedswere extracted from images in which each samplehad been placed in the same orientation (Fig. 1)and measured by digital analysis.

The imaging hardware and software, and theimage grabbing and analysis, have been describedextensively in earlier studies, and the reader is re-


ferred to the relevant publications (Braadbaart2004b; Braadbaart and van Bergen 2005).

The Carbonization Process andLaboratory Simulation

Samples of sunflower achenes and seeds ex-posed to any heat source will absorb the heat orthermal energy that is transferred by radiationfrom the source to the samples. When this pro-cess occurs under conditions where air has freeaccess, samples will burn and ash. Under reducedor anoxic conditions (i.e., the absence of air), thesamples not only undergo a very different chemi-cal conversion, but contrary to the burning pro-cess, their structure is often retained and may berecognized following recovery from archaeologi-cal contexts. This process is usually referred to ascarbonization or charring. The absorption of theheat causes an increase in temperature of thesamples, and the degree of change is dependenton heat source-related variables such as tempera-ture and time of exposure. Heat transferred intoan object by thermal radiation is a function ofseveral components, including surface reflectivity,emissivity, conductivity, surface area, and geomet-ric orientation, among others. This second groupof variables, which also influences the change, isdirectly related to the properties of the sampleand is referred to as the sample-related variables.

Increasing temperature will drive the chemicalreactions that convert the main constituents of theachenes and seeds, polysaccharides (cellulose),proteins, and lignin, into a range of new, mostlyaromatic, moieties, while most oils will evaporate(Braadbaart et al. 2007). These conversions are ac-companied by the release of large amounts ofvolatiles that disappear into the external environ-ment. One result is a mass loss that quickly in-creases to 60 or 70% from 250 to 370°C, andreaches more than 80% at 600°C. Simultaneously,the carbon content increases, hence the term “car-bonization” (Braadbaart et al. 2007). In addition,the color of the achenes began to blacken at290°C. At higher temperatures, the color remainsblack, but other physical properties and the chem-ical structure change as a function of temperatureand time. Accordingly, carbonization is a processthat depends strongly on a number of variables.

In the current study, the influence of thesevariables on the physical and chemical propertiesof carbonized achenes and seeds is investigated bysimulating the carbonization process. In the labo-ratory, samples were heated in a tube oven under

a constant flow of nitrogen to produce anoxicconditions. Samples were heated to a variety oftemperatures and times. More details about car-bonization are described in Braadbaart (2004b)and embedded in the discussions below.

Variables Determining theCarbonization Process of Sunflower

Achenes and SeedsHeat Source-Related Variables

Temperature and the time of exposure are themost obvious heat source–related variables that de-termine the carbonization process. Loss in mass ofthe samples is a key feature associated with this pro-cess. In previous experiments with peas and wheat,Braadbaart (2004b) has shown that samples withan equal mass loss have an identical chemical struc-ture. This important relationship exists becausespecimens absorb a similar amount of heat or ther-mal energy that drives the chemical reactions, andthat, in turn, converts the main constituents of thespecimens into a series of new moieties. For peasand wheat, as well as for sunflower achenes andseeds, these changes occur in two stages. At temper-atures lower than 370°C, or stage 1, mass loss notonly intensifies with higher temperature, but alsowith increasing time of exposure. This scenario im-plies that a sample that has attained a mass loss of40% could be exposed to a heat source for sevendays at 290°C, but the same mass loss could also bereached by heating a sample for 120 minutes at310°C or for nine minutes at 340°C. In the secondstage, with temperatures higher than 370°C, andafter a short initial time (maximum eight minutes),the mass loss remains constant for each tempera-ture even when the time of exposure increases. Inaddition, the chemical structure does not change.As previously described, the reflectance measuredon samples can be used as a proxy for the chemicalstructure. By measuring the reflectance on samplescarbonized at temperatures higher than 370°C, it ispossible to determine the temperature at whichachenes and seeds were carbonized, even if the timeof exposure is unknown.

Above we suggest that temperature and time ofexposure determine the carbonization process.However, one needs to consider “time” as thetime needed to heat achenes and seeds from theirambient to their final temperature. This approachto time is determined by the heating rate or theincrease of the temperature of the samples in de-grees Celsius per unit of time (°C min−1).


The heating rate depends on heat source andsample-related variables. The large number ofvariables that shape the carbonization processmakes it difficult to study the influence of thisprocess on achenes and seeds. To understand theprocess, it becomes necessary to isolate and limitthe number of variables. Consideration of theconditions in which archaeological achenes andseeds may carbonize resulted in our selection oftwo kinds of heating conditions.

In the first scenario, we attempted to simulateconditions where samples come into direct andimmediate contact with the heat source, for ex-ample, when a burning roof falls on stored sun-flowers. For this simulation, achenes and seedswere introduced into a preheated tube oven anddirectly exposed to the prevailing temperature ofthe heat source. Thermocouples inserted withinthe samples measured the temperature within thegrains during the whole experiment, and the re-sults indicate that the samples attained the oventemperature in a relatively short time (Braadbaartet al. 2004a). For oven temperatures of 310 and600°C, this took 11 and 6 minutes, respectively,and corresponds to heating rates of 26 and 97°Cmin−1. For the purpose of this study these valuesare designated as high heating rates (HHR).

A second condition was simulated in whichsamples are less directly and more slowly exposedto a heat source. In an archaeological context,one can think of seeds in a ceramic vessel situatedclose to or in an open fire or in the vicinity of afire that has just started and still has to reach itsfinal temperature. Under this scenario, the tem-perature of the sample will rise from an ambienttemperature of around 20°C, which is assumedto be the temperature before the fire existed, toits final temperature. To simulate these scenarios,the samples were placed in the tube oven that hasreached this ambient temperature. Subsequently,the oven was set for the selected final temperatureand heating rate. By selecting a heating time ofaround 120 minutes, the heating rates are muchlower than the ones attained in a preheated oven,as mentioned above, and are designated as lowheating rates (LHR).

Sample-Related VariablesSample-related variables are directly related to

the size and the chemical composition of thesamples. However, variation in chemical compo-sition associated with thermal properties is lim-ited (Braadbaart et al. 2007), and it is, therefore,

assumed that these variables are identical for theinvestigated varieties. In the case of sunflowers,size may differ according to the variety; conse-quently, three different varieties were investi-gated.

The oil and water content of uncarbonizedachenes and seeds requires more intensive analy-sis. The quantity of free water present in sun-flower achenes or seeds is converted into vaporthat disappears into the external environmentwhen they attain a temperature of 100°C. Theavailable heat is used only for this conversion. Asa result, the temperature of the sample will re-main stable until all the water has been removed.If heat is still available, the temperature will riseagain, and the usual chemical reactions character-istic of the carbonization process will occur. Asimilar conversion, but at different temperatures,occurs for any oils present in the samples. Inshort, our experiments show that variation inwater or oil content of uncarbonized achenes andseeds will not influence the chemical conversionof their other constituents, the polysaccharides,proteins, and lignin. This does not mean that theoil content could not influence mass loss orchanges in dimensions.

Carbonization and Change of Mass,Reflectance, and Dimensions

Selection of the VariablesCarbonization experiments with sunflower

achenes and seeds, variety Arikara, were per-formed at oven temperatures ranging from 160 to700°C. The samples were introduced into a pre-heated tube oven inducing carbonization at aHHR. The time of exposure of all the experi-ments was 60 minutes, which was considered suf-ficient based on earlier works (Braadbaart et al.2007; Braadbaart and Bakels n.d.). These experi-ments afford results as a function of a range oftemperatures. Only the variety Arikara was usedfor the HHR experiments. Subsequently, the in-fluence on the carbonization process on more va-rieties and at different heating rates was investi-gated. Hopi Black Dye and Apache Brown, inaddition to Arikara, were used.

The influence of a LHR was studied by intro-ducing the samples into a tube oven with an am-bient temperature of about 20 °C. Tests were per-formed on samples of seeds and achenes, eachcontaining 10 specimens per variety. To reducethe number of tests for the additional experi-


ments, three temperatures were selected: 310,440, and 600°C. The rationale for the selectionof these specific temperatures is as follows. At310°C the conversion of polysaccharides, lignin,and proteins, present in achenes or seeds, intoaromatic moieties is almost completed. The con-version makes the resulting residues more resist-ant to attacks by microorganisms upon depositinto the soil, and the chances that this materialwill be recovered from archaeological sites aremuch greater compared to material that consistsof starch and proteins. The temperature of 440°Cwas selected because the chemical structure, nowconsisting only of condensed aromatic moieties(the lipids are not present anymore), will notchange at higher temperatures. However, the re-flectance continues to increase. The temperatureof 600°C was selected because it seems to be amaximum temperature that can be reached whensamples are heated in an open fire.

To study the influence of the LHR, the sam-ples were carbonized at the above temperatures inseparate series of experiments. For each tempera-ture, 120 minutes was allowed for the oven toreach the optimal temperature of 310, 440, and600°C. Accordingly, the heating rate varied from2.4–4.8°C min−1. After the desired temperatureswere attained, the samples were kept at that levelfor another 60 minutes in order to compare theresults to those obtained under the HHR regimefor 60 minutes.

Mass Loss of Achenes and SeedsSamples, each containing 10 achenes or seeds,

were weighed before and after carbonization, inorder to calculate the percentage of mass loss. Astrong rate of mass loss was observed for bothachenes and seeds carbonized in the range of 250to 400°C, after which the rate of mass loss slowlydecreased with temperature (Fig. 2).

The main constituents of sunflower seeds arelipids (47–65 wt%) and proteins (20–40 wt%)(González Pérez 2003). Carbonization has differ-ent effects on each biopolymer (Braadbaart et al.2007). Lipids will evaporate, and, at a tempera-ture of 370°C, they have mostly disappeared. At400°C lipids are no longer recognized. Proteinsreact differently. At temperatures in the range of150–250°C, when water once again becomesavailable due to dehydration processes, the pro-teins will swell, causing an increase in dimensions.At temperatures exceeding 250°C, the proteinswill gradually convert into aromatic compounds.


Fig. 2. Mass loss (%) of sunflower achenes and seeds as a function of the temperature in °C. Solid symbolscarbonized at a high heating rate (HHR), open symbols at a low heating rate (LHR).

From 310°C and higher, proteins are no longerobserved. This process is accompanied by the re-lease of many volatiles and, in turn, a strong massloss occurs (Fig. 2). Between 310 and 440°C, afurther condensation of the aromatic compoundsoccurs with a loss of CO and CO2 that results inmore mass loss (Braadbaart 2004b). At highertemperatures, a different type of conversion oc-curs, and the rate of mass loss decreases further.

The mass loss of seeds between 250 and 370°Cis somewhat less than for achenes. With increasingtemperature this difference gradually becomessmaller, and at temperatures above 400°C, themass loss of seeds exceeds that for achenes. Thesedeviations can be attributed to the relative differ-ences in the presence of oils and the fact that oilsdo not carbonize but rather evaporate. Mass lossoccurs by evaporation of oils and chemical con-version of the other constituents. Seeds containrelatively more oils than achenes. Apparently inthe lower temperature range, evaporation of oils isslower than chemical conversion. Consequently,seeds lose less material and have a lower mass loss.Once oils have evaporated, between 370 and400°C, the mass loss of the seeds becomes higher.

When compared to a HHR, the mass loss ofachenes and seeds carbonized at a LHR is lower.The volatiles, which occur as a result of the car-bonization process, traverse the residues on theirway to the external environment, causing second-ary reactions between these volatiles and the con-verting solids of seed and hull. In the case of aLHR, the time in which the volatiles residewithin the heated specimens is longer comparedto a HHR, and more time is available for second-ary reactions. Thus more secondary moieties withidentical chemical structure are formed. Conse-quently, more mass is formed and mass loss de-creases.

ReflectanceThe chemical structure of the constituents of

the different sunflower varieties is similar. Attemperatures where reflectance can be measured(higher than 270°C when aromatic ring struc-tures start to form), the chemical structure of thecarbonized sunflowers will be similar regardless ofvariety. To measure reflectance, the varietyArikara was selected. Reflectance rises slowlyfrom 270 to 400°C (Fig. 3). Thereafter, it risesmore rapidly. The S.D. of the measurements in-creases from 0.072 at 280°C to 0.23 at 600°C.

When samples are carbonized at a HHR or


LHR at identical temperatures, the mass loss mayvary, but the chemical structure is identical and,in turn, the reflectance will be identical. By mea-suring reflectance, the temperature at which ach-enes and seeds have been carbonized can be mea-sured independent of the heating rate (Braadbaartand Bakels n.d.).

Seeds: Changes in Length and WidthSamples containing 10 seeds of the variety

Arikara were prepared by manually removing thehulls from the achenes. Length (L) and width(W) of each seed were measured and their meanscalculated. These samples were carbonized for 60minutes in the preheated tube oven (HHR) ateach of the following temperatures: 160, 220,250, 310, 340, 370, 400, 440, 500, and 600°C.After cooling, the specimens in the samples wereremeasured and new means were calculated.Changes in the means of L and W are plotted asthe percent shrinkage versus temperature (Fig. 4).Shrinkage is defined as the ratio of the untreatedor original dimension minus the carbonized di-mension to the untreated or original dimension,for example, for the width: Wuntreated − Wcarb /Wuntreated * 100 %.

At 160 and 220°C, the Arikara seeds swelled,resulting in increased L and W, which is shown asa negative shrinkage (Fig. 4). From 250°C, Lstarts to shrink, but W remains almost equal tothe width of the untreated seed until 370°C. At

Fig. 3. Reflectance measurements (%Rr) on sun-flower achenes (var. Arikara), carbonized for 60 min-utes at a high heating rate (HHR), as a function of thetemperature in °C. ▫ = samples of achenes.


Fig. 4. Mean shrinkages of length (L) and width (W) of samples containing ten sunflower seeds of the indi-cated varieties as a function of the temperature in °C. Solid symbols carbonized at a high heating rate (HHR),open symbols at a low heating rate (LHR).

higher temperatures, W begins to shrink, eventu-ally reaching a 20% reduction at 600°C. Underthese conditions L shrinks slowly from 250 to310°C, then shrinks more rapidly reaching an18% reduction at 370°C and 28% at 600°C.

Subsequently, samples of 10 seeds of the vari-eties Hopi Black Dye and Apache Brown werecarbonized under similar conditions (HHR), andthe shrinkages of L and W were calculated basedon their sample means. The mean mass of theoriginal Arikara seeds is higher than the seeds ofthe other two varieties, which points to a higheroil content. The results indicate that differencesin shrinkage of L and W between the three vari-eties are not substantial, even when the oil con-tent differs (Fig. 4).

In order to investigate the influence of theheating rate on the shrinkage of L and W, thethree varieties were carbonized at 310, 440, and600°C at a LHR. At temperatures higher than310°C, seeds of all three varieties carbonized at aLHR show a substantial greater shrinkage of Land W than when carbonized at a HHR (Fig. 4).The differences between the three varieties arestill rather small. It is noted that the seeds car-bonized under the latter conditions are rather dis-torted and fragile, but L and W are still measura-ble. One may wonder if these vulnerable residueswill survive deposition into the soil and subse-quent recovery by the archaeologist.

Achenes: Changes in Length and WidthSamples of 10 Arikara achenes were carbonized

under similar conditions (HHR) as described forseeds. The shrinkages of L and W are plotted ver-sus the temperature of the heat source (Fig. 5).From 160°C, the shrinkage of W gradually in-creases to 13% at 310°C, then more strongly to23% at 340°C. Thereafter it increases moreslowly to 27% at 700°C. The shrinkage of L per-sists at about 3% to 310°C, then increases to17% at 370°C, and eventually rises to 22% at700°C.

Samples of the varieties Hopi Black Dye andApache Brown were carbonized under similarconditions (HHR) and shrinkages calculatedbased on the sample means for L and W. Theachenes of Hopi Black Dye show a trend similarto Arikara achenes, but the shrinkage is about 5%less for both L and W. Uncarbonized Hopi BlackDye achenes are the smallest of the three vari-eties, and this difference in size might explaintheir relatively less shrinkage. However, uncar-


bonized Arikara achenes are smaller than ApacheBrown achenes, but their respective percentshrinkage does not substantially differ.

The influence of the heating rate on L and Wof sunflower achenes was determined by carboniz-ing the three varieties at 310, 440, and 600°C at aLHR. For achenes, the results are different fromthose obtained for seeds. The differences amongthe varieties are rather small (Fig. 5).

Properties of the ArchaeologicalAchenes and Seeds

Internal MorphologyThe microphotograph of the polished cross

section of the archaeological achene (Fig. 6a)show depressions (d) that are surrounded by lightcolored walls (w). It is very likely that these wallsare the original cell walls, which have been pol-ished to a flat surface. The SEM micrograph ofthe cross section of an achene of the varietyArikara, heated for 60 minutes at 370°C (Fig.6b), is also characterized by depressions (d) sur-rounded by walls (w), but in this case the wallsdo not exhibit a flat surface. When comparingboth figures, a similar cross section is observed;however, the polishing necessary to measuring re-flectance resulted in a flattening of the superiorwalls of the archaeological achene.

Reflectance and Chemical StructureThe reflectance and chemical structure of the

five archaeological achenes recovered from theStelzer site were determined. The reflectance var-ied from 0.63 to 1.25%Rr, corresponding totemperatures of 365 and 420°C, respectively. Thedifferences in temperature are possible as eachachene will not have been in the same positionnear or within the fire that caused the carboniza-tion. Of great interest, the temperatures derivedby reflectance fall in the range where the greatestvariation in shrinkage occurs (Fig. 5). Also of sig-nificance, and as expected given the results of ourexperiments, oils are not present in the specimenscarbonized at 400°C and higher.

Reconstruction of the OriginalDimensions

GeneralOpen fires, wherein the supply of air is limited

due to a poor mixing of fuel and air (Albini1993), are the commonly expected conditionsproducing carbonized plant remains at archaeo-


Fig. 5. Mean shrinkages of length (L) and width (W) of samples containing ten sunflower achenes of the in-dicated varieties as a function of the temperature in °C. Solid symbols carbonized at a high heating rate (HHR),open symbols at a low heating rate (LHR). Dashed lines indicate the upper and lower limits of the mean shrink-ages.

logical sites in temperate open-air settings. To en-hance analysis, it becomes important to under-stand the carbonization process. Accordingly, car-bonization experiments on modern plant tissuesunder well-controlled conditions in a laboratorywere conducted and compared to experimentswith similar propagules under less controlledconditions in an open fire (Braadbaart 2004b).The results of the experiments under both typesof conditions show that the processes governingcarbonization are similar for any type of heatsource. Therefore, results obtained from experi-ments carried out in the laboratory can also beused for carbonized specimens recovered from thearchaeological record. This conclusion is impor-tant and affords a better understanding of howarchaeological remains have obtained the charac-teristics that today’s researchers observe.

The difference in temperature between a heatsource and the achenes and seeds causes a transferof heat by radiation to these samples; the temper-ature of the samples will increase until it hasreached the temperature of the heat source. Inthe case of oil-containing seeds, the increasingtemperature will cause the evaporation of thelipids and drive the reactions causing the chemi-cal conversion of polysaccharides, proteins, andlignin, the other main constituents of the sam-ples. It is these reactions which result in the lossesof mass noted for the carbonized samples (Braad-baart et al. 2007). Consideration of these factors


provides methods that make possible the recon-struction of the original dimensions of car-bonized sunflower achenes and seeds recoveredfrom an archaeological site.

The results of the present investigation on threevarieties of sunflower achenes and seeds underdifferent carbonization conditions show that tem-perature, heating rate, and variety cause differencesin shrinkage of lengths and widths, varying accord-ing to data presented in Figures 4 and 5. As de-picted in these figures, an important trend is ob-served that corresponds with the mass loss of thesamples as a function of the temperature (Fig. 2).This trend is especially well noted in the case of ach-enes. It is possible to ascertain upper and lower lim-its of shrinkage (note the dashed lines in Fig. 5) thatdescribe the shrinkage as a function of the tempera-ture. By measuring the reflectance, it becomes pos-sible to determine the temperature at which car-bonization occurred, and by comparing thereflectance data to the background data achieved bythe experiments, upper and lower percentages ofshrinkage can be determined for archaeologicalspecimens. Although achenes and seeds show moreor less a similar behavior, certain differences are ob-served that require a separate discussion.

SeedsThe cells of the seeds contain oil as well as pro-

tein (Braadbaart et al. 2007). No other biomole-cules are present, except those that constitute the

Fig. 6. Micrographs of the internal structure of sunflower achenes: (a) polished cross section of the resin-embedded archaeological achene, and (b) SEM cross section of an achene (var. Arikara) carbonized for 60 minutesat 370 °C. d = depression and w = wall.

cell walls. At 310°C, the mass loss of the seedsreaches 30%, but hardly any shrinkage of L andW is observed (Fig. 2 and 4). The width does notshrink until 370°C, when the bulk of the oil isevaporated. These results point to an evaporationof oil from the cells between 310 and 370°C;during the intervals, the dimensions of the seedsremain unchanged because the oil inhibits theshrinkage of the cells. Thus only a gradual in-crease in mass loss is measured, while the shrink-age of L and W is limited. Once the oils havecompletely disappeared, between 370 and 400°C,the amount of shrinkage increases.

Little difference in shrinkage is observed be-tween the varieties when the samples are car-bonized at a HHR for all investigated tempera-tures. Under a LHR regime to temperatures of310°C, the percentage shrinkage of the seeds isidentical to seeds carbonized at a HHR. But athigher temperatures, the shrinkage of LHR seedsis much higher than that for HHR seeds. Figure7 illustrates the results. In the figure, the lowestlevels of shrinkage for the three varieties reflectseeds carbonized at a HHR. Their shrinkageforms a nearly straight line (Fig. 7). Beginning at310°C, the upper range of shrinkage reflectsthose seeds that were carbonized at a LHR. Itshould be noted that the LHR seeds are greatlydistorted and fragile and are not likely to surviveburial and recovery from archaeological contexts.It initially appears that slower heating causes ahigher percentage of shrinkage than quicker heat-ing. However, we suggest that it is not the rate ofheating but the time of exposure to the heatsource that explains the results. At LHR the seedsremained for three hours in the oven, while at aHHR they spent only one hour in the oven. Therelationship with time is supported by previousexperiments by Wright (1998, 2003).

Carbonized seeds are frequently found to-gether with pieces of carbonized broken hull orwith pieces of hull still stuck to the seed. Appar-ently seeds have been carbonized within the hulls,and after deposition or during recovery activities,mechanical forces caused the hulls to break.However, the current experiments reflect seedscarbonized after the removal of the hull. To inves-tigate the influence of the presence of a hull,whole achenes were carbonized. Subsequently thehulls were carefully removed and the seeds mea-sured to calculate the shrinkage. For this purpose,four samples were used: two carbonized at 440°Cand two at 600°C, all at a LHR. The results indi-


cate that the shrinkage of the seeds is a functionof the presence or absence of a hull at a HHR(Fig. 7). Moreover, a linear relationship with astrong positive correlation for both L and W ex-ists. When reflectance is measured and plotted inFig. 7, the shrinkage can be measured and theoriginal dimensions reconstructed.

AchenesThe rate of shrinkage of achenes, as a function

of the temperature, corresponds with the rate ofmass loss: a strong increase in shrinkage is ob-served from 250 to 370°C, and thereafter theshrinkage increases more gradually (Fig. 5). Inthe lower temperature range where there is muchshrinkage, the differences between the varietiesand the heating rates are rather small. It is notedthat seeds behave in a different way. In tempera-ture ranges greater than ca. 370 to 400°C, thelower limit of shrinkage is represented by car-bonized Hopi Black Dye achenes, which are thesmallest specimens of the three varieties. Theupper limit in this temperature range consists ofArikara and Apaches Brown achenes.

Archaeological Interpretations andReconstruction of the DimensionsTo illustrate how the original dimension can

be reconstructed from the dimensions after car-bonization, the archaeological achenes recoveredfrom a site in Missouri are used. The results aresummarized in Table 2. On each sample the re-flectance (%Rr) was measured. By plotting thereflectance in Fig. 3, the temperature at which asample has been carbonized can be measured.Concurrently the temperature of each sample isplotted as a vertical line in Fig. 5, and the rangeof shrinkages can be measured at the intersectionof each line with the upper and lower limit of Land W. The shrinkage for L varies between 7.5and 19% and for W between 16.5 and 29%. Toreconstruct the original L and W, the correctionfactor, 100/(100-% shrinkage), has to be calcu-lated. For L this factor ranges between 1.08 and1.23, and for W between 1.20–1.41 (Table 3). Bymultiplying the correction factors with the actualL and W of each sample, the original L and W isreconstructed. The original length and width isalso reconstructed by using Yarnell’s correctionfactors: 1.11 for lengths and 1.27 for widths. It isimportant to note that for the five archaeologicalachenes, the latter correction factors produce lowvalues for the uncarbonized dimensions


Fig. 7. Relationships between mean shrinkage of length (L) and width (W) of seeds of the three indicated va-rieties and the temperature in °C. The solid and open symbols represent high heating and low heating rates, re-spectively, and are carbonized after removal of the hull. Crosses (+) represent seeds carbonized in the hull at a lowheating rate.

Consequences for ArchaeologicalResearch

In 2001, while arguing for the domestication ofsunflowers in Mesoamerica, Lentz et al. called intoquestion the use of measurement as a means to dis-tinguish domesticated versus wild forms of sun-flower. The measurement scheme (and correctionfactor) referred to by Lentz et al. (2001) was insti-tuted by Richard Yarnell. In his seminal article,“Domestication of Sunflower and Sumpweed inEastern North America,” Yarnell (1978:296) says,“In order to convert measurements of carbonizedsunflower seeds and achenes to estimates of originalachene size, it is necessary to increase achene lengthand width by 11% and 27% respectively, in accor-dance, with results obtained by Heiser (1953) . . .”

Yarnell’s correction factor became a standardfor those researching the domestication of sun-flower in North America. One could increase theachene length and width as prescribed and thencompare the results to Table 1 of Yarnell’s 1978text. There Yarnell indicates that a L and W rangeof 3.0–5.5 × ? (W) is suggestive of a wild form ofsunflower; 4.0–7.0 × ? (W) is suggestive of a rud-eral form; and 6.3–20+ × 3.2–1.2+ is suggestive


of a cultigen. His Table 1 also contains a series ofcorrected lengths and widths from archaeologicalachenes that had been derived from Late Archaicthrough Mississippian period deposits.

Since Yarnell’s article, experiments such asBraadbaart (2004b), Smith and Jones (1990),Wilson (1984), and Wright (2003) have re-searched the effects of carbonization on a diverserange of seeds. The current experiment focusedon sunflower. Experiments were conducted thatdemonstrate that a decrease of 2.5–22.5% in ach-ene length and 10–29% in achene width canoccur, depending on the heating regime and thevariety used (Fig. 5). These figures are based onthe assumption that the recovered specimenswere carbonized between 310 and 600°C. Cor-rection factors can range from 1.03–1.29 forlengths and 1.11–1.41 for widths. We did not in-clude the correction factors for temperaturesbelow 310°C as these samples still contain sugarsand proteins, and their chances to survive micro-bial attack in the soil are rather small.

In general, our correction factors contrast withYarnell’s single correction factors: 1.27 for widthsand 1.11 for lengths. For the sake of argument inTable 4, we present the mean LW as calculated by

Table 2. Results of the determination of reflectance, carbonization temperature, shrinkages,and correction factors for length (L) and width (W) of five archaeological achenes.

Shrinkage Correction Shrinkage Correction Sample Reflectance Temp. range factors range factors

No. (%Rr) (°C) L (%) L W (%) W

1 0.63 365 7.5–14.5 1.08–1.17 16.5–28 1.20–1.392 0.63 365 7.5–14.5 1.08–1.17 16.5–28 1.20–1.393 0.85 395 10–18.5 1.11–1.23 21–28.5 1.27–1.404 0.95 405 10–18.5 1.11–1.23 21–28.5 1.27–1.405 1.25 420 11–19 1.12–1.23 21.5–29 1.27–1.41

Table 3. Reconstruction of the original length (L) and width (W) of five archaeologicalachenes applying correction factors from this study and Yarnell* (1978).

Original L Original L Original W this study Yarnell* Actual W this study Original W

Sample No. Actual L (mm) (mm) (mm) (mm) (mm) Yarnell* (mm)

1 5.14 5.55–6.01 5.71 9.96 11.95–13.84 12.652 5.07 5.48–5.93 5.63 12.00 14.40–16.68 15.243 5.24 5.82–6.45 5.82 10.82 13.74–15.15 13.744 5.95 6.60–7.32 6.60 10.14 12.88–14.20 12.885 6.08 6.81–7.48 6.75 10.46 13.28–14.75 13.28

* Correction factor for L = 1.11 and W = 1.27.

Yarnell (1978:Table 1) for a variety of EasternWoodland sunflower achenes, and the range ofmean LWs for these archaeological achenes basedon our experiments. The comparison shows thatYarnell’s mean LW produce conservative to moder-ate corrections. Accordingly, when comparing themean LW or mean lengths and widths of an ar-chaeological assemblage to the modern measure-ments, the archaeological assemblage could be clas-sified as “wild” or “ruderal” when, in fact, itrepresents a “cultigen.” While Yarnell’s correctionfactors have been useful in identifying trends inachene and seed changes that can be linked to do-mestication, it is important to note that his correc-tion factors reflect rather conservative estimates.

The problem in interpreting sunflower remainsmay not be measuring per se but rather the use ofa single correction factor (viz., increasing achenelength and width by 11% and 27%, respectively)for all achenes and seeds. If one considers thatvariation in size is dependent on achene develop-ment and the variety of sunflower (Nelson 2002)and couples this understanding with variation insize as a result of heating, a single correction fac-tor can grossly bias the interpretation. We argue


that looking at ranges of large assemblages orcaches provides a more realistic basis for interpre-tations for subsistence strategy. In addition, re-flectance can control for the heating regime, andprovides a corrected size range that is more spe-cific in correcting for heating regime variables.Improvements in data analysis offered by thisstudy can enhance our abilities to reconstructprehistoric subsistence strategies and to under-stand interactions of crops and weeds.

SummaryIt is understood that the carbonization of sun-

flower achenes and seeds has become increasinglysignificant given recent discussions in the litera-ture about the origins of sunflower domestica-tion. Lentz et al. (2001) call into question the useof measurements and corrections factors such asthose posited by Yarnell (1978). The former(2001:372) aptly suggest that “carbonization is ahighly variable process dependent on the temper-ature of the fire, exposure time, and moisturewithin the plant tissue.”

Our experiment not only provides data to sup-port Lentz’s suggestion, but also provides a meansto compensate for most of the variables associatedwith carbonization. Temperature, heating rate, andvariety determine the shrinkage of achenes andseeds as a result of the chemical conversion causedby heating under anoxic conditions. By measuringthe reflectance, the temperature at which the car-bonization occurred can be calculated. The nextstep is the calculation of the shrinkages and the ac-companying correction factors to reconstruct theoriginal dimensions of achenes and seeds. Ratherthan abandon correction factors based on car-bonization, we recommend testing the reflectanceof archaeological seeds or achenes as a means to re-construct their noncarbonized dimensions. As acaution, we acknowledge that the sunflower vari-eties recovered from archaeological sites may be-long to unknown taxa or may have been distortedin ways that are unknown.

It is important to note that reflectance has ap-plications beyond discussions of sunflower do-mestication. For example, in Europe, differentiat-ing species and varieties of carbonized wheatgrains (Triticum L.) based solely on the measureddimensions poses problems (Hillman et al. 1996;Jacomet 1987). Experiments involving the heat-ing of three species of wheat have shown that theinitial difference in the ratio of length to widthamong the three species gradually disappears with

Table 4. Mean LW based on correctionfactors from Table 1 of Yarnell (1978) comparedto correction factors as calculated in thisstudy.

Mean LW Range of mean LW from Table 1, based on this study

Sites Yarnell (1978) (see text)

Late Archaic to Early Woodland

Higgs, TN 24 20–31Salts Cave J4: 10–11 18 15–24Salts Cave J4: 5–8 24 20–31Salts Cave J4: 4 25 23–33Mammoth Cave–Nelson 49 40–63

Middle to early Late Woodland

Owl Hollow, TN 34 28–44Boyd, MS 25 20–32

MississippianGreat Oasis, IA 56 46–72Wilford, MS 41 35–53Steed Kisker, MO 58 47–75Paul McCulloch, MO 82 66–106Turner-Snodgrass, MO 76 61–98Campbell, I., OH 78 64–101Cramer, OH 78 64–101

increasing temperature (Braadbaart and vanBergen 2005). Accordingly, the use of reflectancecan be broadly applied to a variety of plants andinvestigations and interpretations that hinge onmeasurements of carbonized remains.

AcknowledgmentsTechnical assistance with scanning electron mi-

croscopy (SEM) by Dr. W. de Priester (ClusiusLaboratory, Leiden University, the Netherlands) isgratefully acknowledged. We thank the anony-mous reviewers for their insightful comments,which made this a better and more accurate paper.

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