1

Accepted manuscript for

Alexiou, A., Müller, N.S., Karatasios, I., and Kilikoglou, V., 2013, The performance

of different adhesives for archaeological ceramics under mechanical stress, Applied

Clay Science, 82, 10-15

Note: this version does not include some minor changes implemented during proofing

of the article. The published article can be found at

http://www.sciencedirect.com/science/article/pii/S0169131713001816

DOI: 10.1016/j.clay.2013.05.017

2

THE PERFORMANCE OF DIFFERENT ADHESIVES FOR

ARCHAEOLOGICAL CERAMICS UNDER MECHANICAL STRESS

Konstantinos Alexiou1, Noémi S. Müller

2, Ioannis Karatasios

2, Vassilis Kilikoglou

2

121st Archaeological Ephorate of Prehistoric and Classical Antiquities, Ministry of Culture, Athens,

Greece (kalexiou@culture.gr) 2Institute of Materials Science, NCSR "Demokritos", Aghia Paraskevi, Athens, Greece

(nmueller@ims.demokritos.gr, ikarat@ims.demokritos.gr, kilikog@ims.demokritos.gr)

Abstract: The influence of environmental conditions, in particular temperature, on the performance of

adhesives joining different archaeological ceramic fabrics is assessed in laboratory conditions.

Different types of model ceramic fabrics were manufactured and joined with commercially available

adhesives, which are commonly employed in the conservation of archaeological ceramics, and a series

of systematic tests on the various adhesive-ceramic systems was conducted. The response of joins to

static mechanical stress was assessed in controlled loading tests. Results of those tests as well as failure

mechanisms observed are discussed.

Keywords: archaeological ceramics, adhesives, mechanical testing

1. Introduction

Considerable efforts have been made in order to identify the ideal adhesive for

archaeological ceramics, which would satisfy the many parameters that should be

considered in archaeological conservation. Requirements of an effective adhesive

include factors such as long-term stability, minimal shrinkage, ease of application but

also health and safety considerations. Furthermore, for archaeological ceramics, it is

important for an adhesive to be removable to a satisfactory degree. Of importance also

is the appropriate level of bond-strength: the bonds should not be stronger than the

ceramic itself so that possible fracture occurs in the adhesive or in the adhesive-

ceramic interface (cohesive failure in the adhesive or adhesion failure) and not in the

ceramic itself (cohesive failure in the substrate) (Horie 1987, 74f). It is for this reason

and in order to ensure optimal adherence, that different ceramic fabrics usually

require different adhesive systems: for high fired, non-porous ceramics or porcelain

with high mechanical strength, thermosetting epoxy resins are usually preferred, while

for lower fired, more porous earthenware, which are usually weaker thermoplastic

adhesives are employed (Cronyn 1990, 152f). Here especially Paraloid B-72 has been

hailed as an excellent adhesive for archaeological ceramics that can also be used as a

consolidant when priming the joints before adhesion is deemed desirable, so as to

strengthen a friable substrate (Koob 1986). For the thermosetting epoxy resins, it must

be noted that their greater strength comes at the cost of ease of removal – epoxy resins

are not readily soluble in most solvents and must be removed mechanically, usually

after softening of the resin.

As noted above, physico-chemical stability is an important parameter for

adhesives used in the conservation of archaeological objects. Polymers are known to

deteriorate over time, a process which involves changes in their appearance and

material properties: for example cellulose nitrate adhesives become yellow and brittle

and lose flexibility upon aging (Koob 1982; Down et al. 1996). While there are

various types of polymer degradation (McNeill 1992) the most common is that caused

by UV radiation and other environmental parameters.

3

The monitoring of the composition of commercially available adhesives is

recommended, as formulation changes may occur - usually without notification of the

manufacturer - that affect the suitability of an adhesive to be used for conservation,

for example affecting reversibility and aging properties (Nel 2009; Down et al. 1996).

Finally, adhesive bonds need to be able to withstand long-term structural loads

without undergoing significant deformation. Large ceramic artefacts such as storage

jars pose a challenge to conservation not only due to their size, but also because of the

increased requirements they pose on adhesives as compared to smaller vessels. Failure

to meet these requirements has been observed to result in shape distortion, self-loaded

deformation, which - when untreated - ultimately leads to vessel collapse. This

problem is aggravated under highly fluctuating environmental conditions, such as

frequently encountered in the Mediterranean, where the storage conditions in

excavations warehouses, or sometimes even in storage rooms of museums can be far

from ideal for conserved archaeological ceramics. This can be problematic when

conserved ceramic objects are exposed to extreme climate conditions, in particular to

temperature fluctuations, which may result in structural deformations of conserved

archaeological ceramic objects. This is illustrated by the example of the

archaeological museum of the island of Tinos, which hosts large Relief Pithoi dated to

the Geometric period (Kourou 2008; Giannopoulou 2010). Tinos belongs to the

Cycladic islands and is situated in the Aegean, a region with dry hot summers and

relatively mild humid winters. Since there is no climate control installed in the

exhibition room to date, temperature and humidity fluctuate in accordance with

external weather conditions. The situation is aggravated by the large light shaft in the

ceiling, which in summer transforms the room to a veritable greenhouse, with

maximum temperatures on hot days well above 40°C. This has caused the joins of the

vessels to loosen and detach from the rest of the vessels’ body under their own weight

(Figure 1). Shellac had been used in the original conservation intervention, an

adhesive which, due to multiple problems associated with its use for archaeological

objects, including difficulties with removal and potential damage to objects, has

largely been replaced by other adhesives today (Koob 1984).

Figure 1: Geometric Relief Pithos, height 2.55 m. Right photograph: detail showing structural

deformation due to loosening of joins.

4

While durability and physico-chemical stability is of importance for adhesives

employed in archaeological conservation, they must also meet requirements related to

their mechanical performance. While many studies have examined the aging of

adhesives under different conditions, the dependence of the mechanical performance

of adhesive-substrate systems on environmental conditions has received less attention

to date. In order to assess systematically the influence of environmental conditions on

the performance of ceramic-adhesive bonding, we tested - through four point bending

tests - the bonding behaviour of four different adhesives which are commonly

employed in the conservation of archaeological ceramics, for their performance at

different temperatures. Two acrylic copolymers, one adhesive based on cellulose

nitrate as well as a thermoset epoxy resin, were tested for their performance with two

different model ceramics, a fine ceramic of low porosity and a coarse, highly porous

ceramic.

2. Experimental

A. Materials

The adhesives tested are currently used in many conservation projects of

archaeological ceramics. The three thermoplastic adhesives tested, which set by loss

of solvent include Paraloid™ B-72, Paraloid™ B-48N and UHU® Hart. In addition a

thermoset epoxy resin, Pattex® power epoxy, which sets by chemical reaction of the

two components, was also tested. Normally, thermoplastic adhesives are used for low

strength ceramics with higher porosity, while thermosetting epoxy adhesives, which

are not suitable for most earthenware, are employed for dense bodied ceramics such

as porcelain or stoneware (Cronyn 1990, 152f; Williams 2002).

Paraloid™ B-72 (manufacturer: Rohm and Haas) is an acrylic co-polymer of

medium hardness (Rohm and Haas 2007). Its glass transition temperature is close to

temperatures that prevail in summer in the Mediterranean (Tg = 40°C) and Paraloid™

B-72 is known to soften at 30-35°C (Podany et al. 2001). Paraloid™ B-72 is a clear,

colourless ethyl methacrylate (70%) and methyl acrylate (30%) copolymer, with a

high stability against degradation under normal exposure conditions and is employed

for the conservation of many materials (Horie 1987, 106ff). It is strong and tough

without being brittle and has been recommended for use with archaeological

ceramics, both as consolidant and as adhesive (Koob 1986).

Paraloid™ B-48N (manufacturer: Rohm and Haas), a methyl methacrylate

copolymer, is popular among metal conservators in Greece because of its excellent

adhesion to metals. It is occasionally also used for reconstruction of archaeological

ceramics. Paraloid™ B-48N is advertised for its toughness and flexibility and has a Tg

of 50 °C (Rohm and Haas 2007b).

UHU® Hart (manufacturer: UHU GmbH & Co KG) is a transparent

thermoplastic based on cellulose nitrate and has been used for reconstructing

archaeological ceramics in Greece for decades. It is reported to have a Tg of around

50˚C (Nel 2009). While this adhesive is reversible, quick drying and - since it is

commercially available in tubes - convenient to use, cellulose nitrates have a poor

long-term stability. They degrade at room temperature, a process which is accelerated

by light and is accompanied by yellowing as well as the adhesive becoming brittle

5

(Selwitz 1988; Horie 1987, 132). Cellulose nitrates are therefore not counted among

the best choices of adhesives for archaeological objects (Koob 1982).

Pattex® Power Epoxy Saldatutto Instant 5’ Mix (manufacturer: Henkel), finally,

is an epoxy resin, two component system. Epoxy resins are durable, have outstanding

mechanical strength, low shrinkage, and hold joins strongly together as they have

good adhesion to many substrates (Horie 1987, 170 ff; Selwitz 1992). Unlike the

other adhesives tested, once hardened it cannot be removed easily by solvents but

must be removed mechanically, a process which can be helped by softening of the

resin, achieved by applying - normally harmful - chemical solvents or paint removers

(e.g. Davison 2006, 226; Maxwell 1978).

The ceramic model materials were manufactured using a commercially available

calcareous clay (Aegina-type Majolica, procured from the Hellenic Clay Center S.A.)

(CaO c. 23 wt%). The raw clay was used, both untempered and mixed with quartz

sand (screened to grain size of 0.5-1 mm) and pine needles, cut to a length of c. 5-8

mm, to form briquettes. For the tempered ceramic, the quartz : pine : clay ratio was

roughly 1 : 1 : 4 in volume. Briquettes were dried for at least two weeks at ambient

temperatures and then fired to 900°C for one hour in oxidising atmosphere. The

untempered, fine ceramic (AEG) has a relatively low porosity, while high porosity

was achieved in the coarse quartz and pine needles tempered ceramic (APQ) through

burning out of the pine needles (Figure 2).

Figure 2: Cross-sections of ceramic model materials: AEG (top) and APQ (bottom). In the coarse AQP

ceramic, quartz temper grains as well as macro-pores from burnt out pine needles are clearly visible.

The preparation of ceramic-adhesive test bars involved the cutting of the ceramic

briquettes in bars of dimensions of c. 1 x 1 x 6 cm. The bars were cut in half, and after

grinding the fresh surfaces they were joined with adhesive according to the

manufacturer’s instructions. For the acrylic copolymers acetone was used as solvent.

Acetone was applied with a brush on the surfaces of those samples that were adhered

with Paraloid™ B-72, Paraloid™ B-48N and UHU® Hart in order to aid capillary

action and enable deeper penetration of the adhesive into the pores. The joints were

left to dry completely before testing. Environmental temperature for joining and

drying was c.30°C.

6

B. Mechanical testing

The performance of the ceramic-adhesive composites was assessed on an

INSTRON universal tester on four point bending tests (Figure 2). The measurements

were performed at 30°C, as well as in a cold environment (c. 0°C) and a hot

environment (50-55°C). The loading rate was held constant at 100 µm/min and load -

displacement curves were recorded for every specimen. For each adhesive - ceramic -

temperature set 5 specimens were tested (Table 1), while the ceramic material itself,

without adhesive, was tested on 3 specimens. Relative humidity was between 50 and

60% for all experiments.

Test temperatures

c. 0°C c. 30°C c. 50-55°C

Paraloid B-

72 AEG APQ AEG APQ AEG APQ

Paraloid

B48N AEG APQ AEG APQ AEG APQ

UHU Hart AEG APQ AEG APQ AEG APQ

Pattex AEG APQ AEG APQ AEG APQ

Table 1: Matrix table of the test schedule: four adhesives were tested for two different ceramic model

materials, a low porous fine ceramic AEG and high porous coarse ceramic APQ, at three different

temperatures.

Fracture strength σf (in MPa) was then calculated using the following equation

(BSI 2002):

2

21max

2

)(3

bd

ssPf

(1)

where Pmax is the maximal load at fracture in N, (s1-s2) the difference in the span

support rods, b the width and d the height of the specimen (all in mm).

Figure 3: Set-up of the four-point bending test used for the determination of the fracture strength. s1

was held at 40.8 mm, s2 at 6.0 mm for all experiments.

7

3. Results and discussion

In addition to the ceramic-adhesive composites, also the ceramic material itself

was tested for its transverse fracture strength. The untempered low porous ceramic

AEG shows a transverse fracture strength of 16.6 ±1.6 MPa while the ceramic

containing quartz temper and manufactured porosity from burned out pine needles is

significantly weaker with 4.0 ±1.1 MPa (Figure 4). This is expected because both

pores as well as aplastic inclusions are known to decrease a ceramic's strength (see

e.g. Kilikoglou et al. 1995, 1998; Müller et al. 2010). While the untempered low

porous ceramic show completely brittle fracture, in the tempered high-porosity

material some energy is absorbed during crack propagation and a semi-stable fracture

mode is observed.

When looking at the performance of the ceramic-adhesive composites at 30°C

(Figure 4) one can observe that the acrylic adhesives seem to fail at similar loads,

while the epoxy, as expected, is significantly stronger. Important for the performance

of an adhesive, however, is not only the fracture strength of joins, but also how

fracture occurs. In the case of the low porous high strength ceramic (AEG),

Paraloid™ B-72 and Paraloid™ B-48N both perform similarly: examining the

specimens after testing, it is observed that they fail in the adhesive and at the

adhesive-ceramic interface. UHU® Hart and the Pattex® epoxy, however, do not

leave the material intact, and fracture of the ceramic occurs with some minor loss of

material (Figure 4). For the low strength, porous ceramics (APQ), Paraloid™ B-72

performs well, with fracture leaving the joins intact. Adhesion of Paraloid™ B-48N to

the ceramic appears greater, resulting in some minor material loss as fracture occurs

partially in the ceramic in a few cases. Also UHU® Hart and the epoxy result in

fracture of the ceramic, with in most cases only partial fracture of ceramic for UHU®

Hart, while the epoxy appears more detrimental.

Figure 4: Fracture strength of ceramic-adhesive joints at 30°C and examples of specimen fracture.

8

In addition to differences in strength also different fracture modes are observed

for the different ceramic-adhesive composites: while ceramics joined with Pattex®

epoxy generally show brittle fracture, samples joined with acrylic adhesives show a

more stable fracture. Examples of load-displacement curves of the four different

ceramic-adhesive systems at 30°C are depicted in Figure 5 for the low porous high

strength ceramic.

Figure 5: Load-displacement curves for the four different ceramic-adhesive systems at 30°C for the low

porous high strength ceramic. The inset shows a typical load-displacement curve of the ceramic

material itself.

In terms of the influence of temperature on adhesive performance, perhaps the

most striking is Paraloid™ B-72. At 50°C the adhesive failed at very low loads, in

some samples even before applying load. As could be expected from its low glass

transition temperature, Paraloid™ B-72 performs poorly at elevated temperatures.

With lower temperatures, however, adhesion becomes much stronger. This

phenomenon is illustrated in Figure 6 for the tempered porous ceramic APQ, but is

equally observed for the non-tempered fine ceramic AEG. In fact, at 0°C, the bonding

of Paraloid™ B-72 with the ceramic is strong enough for fracture of samples to occur

occasionally in the ceramic rather than in the adhesive or adhesive-ceramic interface.

At higher temperatures failure did not result in significant loss of material.

9

Figure 6: Performance of ceramic-adhesive composites under four-point bending at different temperatures for the

APQ ceramic.

The other adhesives showed much more stable behaviour when tested at different

temperatures, with no significant differences in strength (Figure 6). Paraloid™ B-48N

shows stable performance at different temperatures, although a small loss in strength

can be observed at higher temperatures, in particular for the highly porous ceramics,

although much less pronounced than for Paraloid™ B-72. In terms of failure, for the

high strength ceramic AEG, failure at all temperatures occurred in the adhesive,

adhesive/ceramic interface. In the weaker porous ceramic APQ, however, it failed to

leave the joins intact. Material loss was relatively minor at higher temperatures, but

substantial at low temperature. Ceramics adhered with UHU® Hart, did not show

significant changes in fracture strength either at high or at low temperatures.

Perceptible, although mostly partial, material loss only occurred in the low strength

porous ceramics at all temperatures. Also for the epoxy no influence of temperature

on fracture strength was found. The mechanical tests were devastating for most of the

samples with epoxy: since adhesion strength is very high, fracture was observed to be

destructive and occurred though the ceramic substrate.

In terms of fracture modes, at 0°C a tendency to brittle fracture is observed for all

ceramic-adhesive systems. At higher temperatures, the ceramics joined with acrylic

adhesives show a more stable fracture, reflecting increased adhesive plasticity, while

in samples joined with epoxy, in general, also at higher temperature brittle fracture is

observed. Figure 7 compares load-displacement curves for the AEG samples joined

with Paraloid™ B-72 at different temperatures.

10

Figure 7: Load-displacement curves for the low porous high strength ceramic (AEG) joined with

Paraloid™ B-72 at different temperatures.

4. Summary and conclusion

In specimens where Paraloid™ B-72 has been employed, fracture through the

substrate occurred only in a few instances when testing the porous ceramic at 0°C,

where a brittle fracture mode is observed. At higher temperatures, Paraloid ™ B-72

shows considerable plasticity: this is reflected in the load-displacement curves, where,

after fracture initiation, the load drops slowly as the joins are opening up while still

being held together by the adhesive. Although having no or only very minor loss of

material, the adhesive strength at elevated temperatures is very low for Paraloid™ B-

72, however, rendering it unsuitable for employment in hot climates when

temperatures are not carefully monitored and controlled. UHU® Hart and Paraloid™

B-48N have performed similarly considering their fracture strength, with UHU® Hart

being slightly stronger at higher temperatures. Specimens with UHU® Hart usually

fail in a more brittle way, whereas at 30° and 50°C the more elastic Paraloid™ B-

48N, in some cases, still holds the joins together even after maximum load is reached,

and can undergo some bending without complete failure. Nevertheless, in the low

strength, high porous ceramic AQP, even at the higher temperatures, fracture is

detrimental and occurs through the substrate in several samples. At 0°C, also Paraloid

™ B-48N exhibits brittle fracture. The adhesion of the Pattex® epoxy finally, was

excellent, resulting in high fracture strength in the bending test. However, apart from

the fine, low porous ceramic at 30 and 50 °C, significant loss of material occurred in

all other cases. For all specimens brittle fracture was observed.

11

The choice and employment of an adhesive must be carefully considered.

Importantly, there is no universal adhesive that can be used for all ceramics and under

all conditions. Amongst other factors, the performance of joins depends not only on

nature of both the adhesive and the ceramic to be joined, but also on the conditions to

which the ceramic adhesive composite is exposed to during further storage. The

choice of adhesive should therefore take into account the nature of the ceramic

material and environmental conditions the conserved object will be exposed to,

alongside the adhesive's performance and aging characteristics. Systematic testing

such as outlined in the present article can help such considerations and assist in the

choice of adhesive, while further work including the assessment of other mechanical

strains, notably shear stresses (Bradley 1984; Shashoua 1993), is required to obtain a

more comprehensive picture, as is an assessment of the influence of ageing on

fracture behaviour. Finally, the effect of environmental constraints on long term

subcritical stresses on adhesive bonds (creep) should also be assessed.

Acknowledgment

We would like to thank G. Vekinis for help with mechanical testing.

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