Paraloid™ B-72 in Fossil Preparation

Paraloid™ B-72 is one of those magical substances in fossil preparation that we don’t know how we could live without. Paraloid™ B-72 comes in little pellets which are dissolved in a solvent to make a liquid. When this solvent evaporates, the Paraloid™ itself is left behind, essentially plasticising whatever it is in contact with.

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If you need to get rid of it, it redissolves in solvent. Depending on the concentration of the solution, it can be used for a multitude of things. It’s very strong, makes a lovely varnish and is also reversible. It also doesn’t degrade over time, doesn’t turn yellow, and has a degree of elasticity to cope with stresses and strains. It also has an indefinite shelf life in pellet form, and an extremely long shelf life when mixed if in a suitable container. What’s not to love?

We’ll break down what it is, how to use it and how to mix it up yourself at home. If this isn’t your cup of tea, we have a range of premixed Paraloid™ solutions in acetone that are ready straight out of the bottle. We also supply a range of precision dispensing bottles for different purposes, perfectly suited to storing and applying Paraloid.

 Contents

Uses of Paraloid™ B-72

what-is-paraloid-used-for.jpg WHAT IS PARALOID™ B-72 USED FOR?

 In fossil preparation, there are 6 main ways that we can use Paraloid™ B-72.

  1. An adhesive (thick and viscous solutions create a strong yet reversible glue)
  2. A consolidant (this means impregnating the rock with very thin solutions - if you have a crumbly rock it soaks in and essentially plasticises it)
  3. A gap filler (either used alone or mixed with matrix or pigment to fill cracks and voids)
  4. A protective barrier to prevent oxidation (useful for preventing pyrite decay)
  5. A protective barrier for archival marking (if you need to write collection numbers on specimens)
  6. A varnish – In museums, when you see fossil bones they often have a certain sheen to them. That’s probably Paraloid™. It brings out the colour of the fossil and provides a soft gloss effect without being too shiny, and can be used on porous or non-porous surfaces.

 Outside of fossil preparation, there are a multitude of uses, particularly in conservation. It is widely used by archaeologists, metal detectorists, in historical artefact and artwork conservation. It can be used on fossils, rocks, ceramics, wood, porcelain, metals (silver, bronze and more), coins, leather, paper, cloth, plaster, paintings, taxidermy and much more. You should avoid using Paraloid™ B-72 on things that react with the solvent the Paraloid™ is dissolved in, such as many plastics and rubber. The solvent should be considered – for instance if using on a painted surface acetone is not recommended.

Paraloid B-72 is dissolved in a solvent (more on these later), with certain concentrations being suitable for different jobs:

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Paraloid B-72 is technically an ethyl methacrylate/methyl acrylate copolymer, and sometimes called an acrylic resin, or an acrylic co-polymer, or a thermoplastic resin. It comes in clear little pellets which can be dissolved in a solvent such as acetone. It is a thermoplastic resin, which means it can be affected by temperature. It has a high glass transition temperature (40◦C) – which means that under the majority of environmental conditions it will not slump or budge.

Palaeontologists and archaeologists have favoured it as a substance for years. As the story goes it will glue a pot back together beautifully, but unlike superglue, although it is a strong bond, if you drop said pot it will break along the lines that were already broken. We haven’t tested it, but it gives you an idea of the power of Paraloid.

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MIXING A PARALOID SOLUTION

Paraloid B-72 comes in pellet form, which is then dissolved in a solvent (a chemical that dissolves other chemicals). The most used solvent by fossilers is acetone, but you can also ethanol or toluene (or certain mixtures of these where safe and appropriate) amongst others (we’ll discuss the pros and cons of different solvents next).

You can dilute the Paraloid solution to the desired concentration for the job – super thin and very low viscosity for consolidation via capillary action and wicking or extra viscous to work as a strong glue. These are some common concentrations (see the table above). The concentration you need will be a process of trial and error. Always test on a bit of rock you don’t mind so much until you become more familiar with how Paraloid behaves on different substrates.

Which container to use?

You need to be careful what you use to mix the Paraloid in. Many plastics will melt or be destroyed by the solvent (e.g. PET). Glass jars don’t react with solvents and so are a great candidate (check the lid for plastics first). HDPE (high density polyethylene plastic) is also a good candidate as it has extremely low reactivity with many solvents. To prevent the lid from sticking (the Paraloid will do its best to do this!), run a little plumbers tape round the threads of the container.

Suspending the Paraloid Pellets

When Paraloid dissolves it can turn into a mass of resin at the bottom of the container. It dissolves just enough to adhere to itself, creating a gloopy lump that has very little surface area (see pictured jam jar on its side, with the Paraloid very firmly stuck to the base!). The best way of mixing a good, even solution of Paraloid is to suspend the pellets in the jar. Only the most dilute solutions are able to dissolve without suspension. 

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There are two approaches for this.

  1. A square of muslin or cheesecloth tied up in a small bundle with thread works great – a bit like a teabag dangling! A piece of thread running under the lid of the jar or container gets trapped holding the baggie up.
  2. Alternatively, we have always used a tea strainer fixed to the lid of a jam jar which we find a little less fiddly. The average tea strainer takes about 10g of pellets. Obviously, if you need a higher concentration, use a bigger tea strainer or top up the pellets when the last lot has dissolved. Pictured here is a rather rustic looking jam jar with a hole punched in the lid to put the chain of the tea strainer through, sealed up with Milliput epoxy putty (another handy thing that many preparators already have in the workshop!). Ours has had electrical tape – probably not ideal but it still works!

Once the pellets are suspended, leave them for a few hours or overnight to gradually dissolve. The higher the concentration of solution, the longer it will take. Some people use magnetic stirrers to constantly agitate the solution which speeds things up, but isn’t necessary.

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Mixing up a weight-by-volume or weight-by-weight solution at home does not need to be complicated. It's relatively simple to calculate the % concentration of Paraloid B-72 in a solvent.

A few percent error here or there can make a real difference. The difference between how a 2% and a 5% solution works is enormous, so therefore it’s worth trying to be as accurate as possible.

You’ll probably have heard about weight-by-vol% solutions – and this is enough to give anyone who hasn’t done Chemistry since they were 15 the collywobbles. Fear not! It’s not as tricky as it sounds to get the right concentration of solution for the job. A 10% wt/vol solution is the same as 10g of Paraloid B-72 for every 100ml of solution. This should not be confused with 10g of Paraloid added to every 100ml of solvent (this is a weight-by-weight solution). 

This can make the brain hurt a bit, but there are two ways to approach the measurements.

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Calculate your proportions w/w. E.g. 10g of Paraloid in 100ml of Acetone is a 10% w/w solution. You might need to experiment as most of the recommendations you’ll find online pertain to w/vol% solutions (or perhaps w/w solutions that the author believes to be a w/vol solution – you can see it is a bit of a wormhole..!). There is no problem doing it this way if you find it easier – just be consistent with yourself and make sure you write on the bottle the concentration.

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Most recommendations are expressed as weight by volume and so you’re more likely to get the results you’re after by following this method. You can make up a solution with a set amount of Paraloid and dilute it to the concentration you require once the pellets have dissolved in the solvent.

      • To do this, mark on the container you’re using the 100ml, 200ml, 500ml or whichever markings you need. Use a measuring jug and mark the height of water in the container to work this out.
      • Dry out the container and you’re ready to go.
      • If you want to make a 500ml of 10% solution, you could suspend 50% of Paraloid in the jar and add say 300ml of solvent (under the necessary quantity by some way so gives a bit of wiggle room).
      • Once the Paraloid has dissolved, you can add more acetone to fill it up to the 500ml mark and give it a good stir.
      • Only then can you be sure that you have exactly a 10% solution.
      • If you accidently make too much solution, you can leave the container open and evaporate some of the solvent away until it comes back down to the 500ml level. Please be safe if you do this – the fumes should not be inhaled – especially if working with Toluene. 

To clean the container you have mixed it in, fill immediately with water before it dries. You’ll then be able to peel away any residue like PVA from your fingers (a childhood favourite of many). To clean the tea strainer, dipping it in acetone is a good idea. We always have a jar of slightly mucky acetone set aside for this purpose (used repeatedly for cleaning or has bits of muck in it).

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Paraloid in its pellet form has an indefinite shelf life. When in solution, and in the right container, Paraloid B-72 solutions will store for many years. Glass jars are excellent (remember to check the lid doesn't have a weird rubbery seal first!), as are nail polish bottles for very small quantities (bonus brush in there too).

HDPE bottles work well. Although expensive, Nalgene bottles are more resistant overall. After a few years there may be minor deterioration, but this will be minimal. HDPE is certainly a safe storage option.

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 1. Acetone

Acetone is the most commonly used solvent, partly because of its accessibility and partly because of its ability to evaporate very quickly (thus leaving the set Paraloid behind). It is the most volatile of the solvents commonly used.

If you’re trying to penetrate very deep into a fossil, this quick drying time can be a hinderance. Even with a very dilute solution that can travel via capillary action (wicking) you may find that it doesn’t soak all the way through to where you need it. Rapid evaporation tends to draw the polymer back to the surface (known as reverse migration) – this can be mitigated by covering or containing the specimen in a polyethylene bag or container, or by working in a cool area. Pre-wetting the specimen with pure acetone or a very dilute solution, before applying a more concentrated solution can help with this.

  • Most popular and commonly used
  • Quick drying (great for use as an adhesive)
  • Reverse migration and lack of penetration possible when in low concentrations due to rapid evaporation
  • Semi-gloss finish (glossiness dependent on concentration and substrate)
  • Easily obtained and mixed
  • Amongst the safest of solvents to use
  • Not well suited to working in hot conditions (e.g. 35 degrees plus)

 2. Ethanol (ethyl alcohol)

Ethanol is less volatile than acetone and therefore can slow the drying time and help prevent reverse migration. It can be mixed with acetone (5-15%) to modify volatility and can act to reduce bubbling, stringiness and increase working time. However, the combination of the two solvents has a higher toxicity than either of them alone so precautions must be taken.

Pure ethanol solutions can cause a slight milky film on the outside of the specimen. Ethanol can be used to consolidate, the outer layer removed with solvent, with an acetone solution applied for a more pleasing finish and seal.

  • Slower drying time (not good as an adhesive)
  • Lower volatility (reverse migration less likely in consolidation but drying times become excessive)
  • Can be mixed with acetone to increase working time of acetone, reduce stringiness and make it more appropriate for working in hot conditions.
  • Paraloid B-72 is not as soluble in ethanol
  • Less glossy finish

3. Toluene

Toluene is less commonly used as a solvent, although it does have its application in some consolidation settings. It is perhaps too slow drying for most situations in fossil preparation, and for us, we consider the toxicity not worth it. It is a colourless substance derived from hydrocarbons, used in some types of paint thinners and permanent markers (the smell is quite distinctive).

However, it is sometimes used as a recreational inhalant and can cause severe neurological harm and so should only be used with due caution with a fume hood. With high toxicity, it is perhaps not the most favourable solvent when there are other options available. There are other options which in the majority of situations work better. Toluene is better suited to conservation of paintings (which may be destroyed by acetone) and wood.

  • Very slow drying
  • Very toxic
  • Least glossy finish

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There are lots of uses for Paraloid B-72 in fossil preparation. It is an incredible versatile material.

One thing to remember is that you should always make sure the specimen is completely dry. Paraloid doesn’t like damp environments. It can create a milky film when used on wet or damp surfaces and go all weird and gummy. 

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We use very thin solutions of Paraloid B-72 to consolidate or impregnate rocks and fossils, essentially turning them to plastic. Repeat applications of 2-5% wt/vol solutions in acetone are frequently used to consolidate, followed by a 5-10% wt/vol solution to seal the surface.

Consolidation can either be done at home or in the field (where you find the fossil). With some fossils, it is impossible to extract them without first consolidating them. Have you ever tried to lift a fossil from the rock only for it to crumble to fragments? Or to fall aparts as soon as it dries out?

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These ammonites preserved in the mudstone are amongst one of the first and most frequent things that can be found when you start fossil hunting. They are usually left by collectors as they are after the more prized crystalline pieces. These stones, once you get them home, usually lead to disappointment when they disintegrate into dust. Paraloid B-72 is the perfect solution to stabilise and preserve mudstone fossils, as it seeps into the porous rock and plasticises it, allowing for it to be kept indefinitely. ©Jessica Winder - thank you for the use of these photos!

Consolidation works best on more porous rocks, which is pretty handy as these more porous rocks are often the ones that require consolidation, such as many (but not all) sandstones, shales, siltstones, chalks and mudstones. Limestones vary a great deal in porosity. A good rule of thumb is if you get it wet, can the moisture seep into the middle of the rock? If so, it is an excellent candidate for consolidation using Paraloid. Otherwise, you can consolidate fine and hairline cracks in less porous rocks. 

As we mentioned earlier, the main difficulty encountered with consolidation is reverse migration – the process by which the solvent evaporates so quickly the Paraloid is pulled to the outside of the specimen rather than the inside. One way to get around this is to wet the specimen with acetone first, and another is to add a touch of ethanol to an acetone solution (5-10%) to slow the drying time (but this comes with increased toxicity). 

Another point to consider is whether it is essential the middle of the specimen is consolidated? Would consolidating only 2-3cm into the specimen with a dilute acetone-based solution be sufficient to provide the strength you’re after? In many situations, this is sufficient and no further steps need be taken.

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Dinosaur eggs are extremely crumbly in the field. The eggshell is very thin and very poorly attached to the rock within that holds the shape. Egg nests require repeated consolidation in the field (left) and then during preparation (right). The eggshell is so loosely attached that they would disintegrate if extracted without a consolidant. For dry settings, Paraloid B-72 is the go-to. In wet settings, Primal Rhoplex WS-24 (available here) is a better option at least in the field. Photographs: Mihai Dumbrava

Keep reading to find out how to apply Paraloid B-72 solutions to consolidate specimens. 

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Paraloid can be a very powerful adhesive with a high tensile strength. When fully dried, it is comparable in strength to some cyanoacrylates. However, it also has a degree of elasticity and flexibilityThe best way to use it as a glue, is to use an extremely high concentration (50-100%) mixed with fumed silica to increase the viscosity and rheology.

To glue two parts together, apply a thin layer to both surfaces to be bonded and then repeatedly pull apart and rejoin them until the adhesive grabs very well. This is called developing tack. This is best done on very sturdy fossils with a good solid surface. On more porous rocks simply applying the thick adhesive and holding or clamping together (where suitable) is sufficient.

A thick, acetone-based adhesive is usually desirable as it dries more quickly than lower concentration or other solvent-based alternatives. Please be aware that some other products on the market contain nitrocellulose which is known to yellow, crack and go brittle (aside from its fame as an explosive, it is commonly found in nail polish remover). Our extra-thick Paraloid solution is pure, containing only Paraloid B-72, acetone and fumed silica.

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These photographs show the application of Paraloid B-72 adhesive (80% wt/vol) to the fossil, and then the pulling apart and pushing together motions that develop tack. You can see those stringy lines between the two parts of the ammonite - these are strings of Paraloid. It will get to a point where it is really difficult to pull the two pieces apart.

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Paraloid B-72 is on the glossy side. The finish could be best described as a soft gloss, usually a bit more than ‘satin’, but not ‘wet look’.  Paraloid B-72 also helps bring out the colours and contrastThe more porous the rock, the less the sheen.

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The sheen can be greatened or lessened depending on the solvent, the concentration and added chemicals. The trilobite pictured has been varnished with Paraloid B-72 (which has also been used for some pretty undetectable gap filling!), with a 10% solution (to be fair it had been sitting around for a while and so was probably closer to 12%) the sheen is higher than that of the dinosaur bones also pictured which weren’t varnished as such but frequently consolidated throughout their preparation.

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Here is a crinoid (before and after) with Paraloid being used as a varnish. You can see the full preparation of this crinoid here. As you can see, when the Paraloid solution goes on it has a super wet look. This is as a result of the acetone. When this evaporates and the Paraloid sets, the rock is left with a soft sheen. If there are areas where you don’t want this to happen or you don’t like the finish, simply dab with acetone on a piece of kitchen roll.

If you need to mattify the Paraloid, you can add a little fumed silica. Approximately 1cm3 in 100ml is sufficient. Any more and the solution can start to become translucent. Alternatively, some artist's matt varnishes may work on top of the Paraloid (retaining a protective layer between fossil and substance) but you would have to be careful of solvent compatibility. A further option is using a very gentle air abrasive like sodium bicarbonate to just slightly mattify the surface of the paraloid, without damaging the fossil underneath. 

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 Paraloid B-72 is an effective solution for gap filling. It can be mixed with ground matrix or colour matched to the fossil to provide a removable plug to help structurally support any missing areas.

Mixing Paraloid B-72 (say 50%-60% solution) with microballoons (tiny hollow glass bubbles used as a lightweight filler in resins) or Cab-O-Sil creates a bulking agent. This can be achieved by using our extra-thick solution which is already combined with fumed silica and diluting it a little. The vast majority of gap or void fillers are not reversible (such as milliput epoxy putty). You typically end up with a translucent filler that can be lightly pigmented or combined with rock dust.

It is a little harder to sculpt and sand than permanent putties, but the surface can be cleaned and smoothed with acetone (taking care not to introduce so much acetone that it weakens the joint that you are filling. It may take a couple of applications, as when the acetone evaporates, it can leave vacuities in the void which will need to be filled. With the right balance, cabosil and paraloid can be used to repair calcite whilst maintaining a level of translucence. Using ethanol gives you a bit more working time.

You can use the thermoplasticity of Paraloid to your advantage – a hair dryer or heat gun can be used to adjust joins and increase working time (check first that the specimen can withstand heat).

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While we won’t go too into detail with archival marking, it is something used in private as well as museum collections. This is the process of marking fossils with either collection numbers or information to identify them or cross-link to a catalogue. So how do you go about writing on fossils without damaging them?

Paraloid can be used to create a basecoat and a barrier between fossil and ink. A 10% solution in acetone can be used to create a smooth surface to write on. From there, using an archival grade technical pen or white ink you can write what you need to, wait for it to dry and then apply a thin layer of Paraloid to protect the ink from sunlight and abrasion, making it more durable. Paper is often preferable to many people as it is easier to write on! Write the catalogue number on a flat surface and attach with a thick Paraloid solution. Coat the surface of the fossil, apply paper, and then coat the top surface of the paper. You can use archival grade or acid free paper (Japanese rice paper and Bristol Board are favourites). A 50% solution is useful to adhere the paper to the surface of the rock. 

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Preservation of pyrite is always a contentious topic and there is no absolutely perfect way of doing it with a genuine and consistent vacuum. One of the things you can do with a pyritic specimen is to seal it off from the air as best as possible as it is oxygen and moisture that cause the reaction leading to pyrite decay. There is no guarantee that it will preserve the specimen, but it is one of the easier DIY solutions that you commonly come across.

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A very thin solution of Paraloid B-72 (5% or less) can be used in a couple of coats to penetrate the specimen a little, and then a thicker solution of 10% can be used to make more of a seal from the atmosphere (pyrite decay being triggered by the presence of oxygen and humidity). Some people will choose to use immersion techniques. The only thing to really mention is that if the specimen has been soaked for a time in water to remove salt, wait for it to dry fully before applying Paraloid. When in the Paraloid solution, make sure to flip and turn it occasionally as you will get air bubbles in there. For this reason, it is often best to coat with a paintbrush rather than immerse, taking care to prevent reverse migration as best as possible, perhaps soaking in acetone first.

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There are a few different ways to apply Paraloid:

Pouring the Paraloid B-72 on – usually done in emergency situations in the field! We supply our premixed solutions in squeezy bottles with spouts for this purpose – they allow significantly more control than straight up pouring.

Precision Application:

We have a range of dispensing bottles (which can be used with Paraloid solutions or other substances), which come with interchangeable twist-to-lock tips or varying diameters for different applications. These tips can be cleaned with acetone and used repeatedly. Our Extra-Thick Paraloid Adhesive is supplied in these bottles – so feel free to reuse and repurpose them as much as you can when the Paraloid is finished!

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These are useful both in the field and when prepping a specimen. Often you pen a little pit, and consolidate, pen a little, and repeat. For instance, when preparing a fish skeleton you may need to consolidate each scale as you go. These dropper bottles are perfect for that as you can be extremely precise where you apply the Paraloid. Because of the very small diameters the contact with the atmosphere is minimal – and so they can be left open for periods of time without too much evaporation.

Brushing

Repeated brushing of a surface is suitable to apply the Paraloid. You have to be quite quick because if you apply too little, especially if your goal is consolidation, the acetone will evaporate and the Paraloid will create a barrier layer on or close to the surface preventing further consolidation.

Use a paintbrush that works with the solvent – wooden handles are best when working with acetone and ethanol. It doesn’t need to be anything fancy – just try to use the right shape and a reasonable enough quality that it doesn’t leave a trail of bristles.

We also carry twist-and-lock brush heads to fit our Precision Applicator bottles.

When brushing Paraloid on, you can encounter two problems

1.    The first being that leaving a jar of solution open whilst using it, it evaporates changing the concentration

2.    If your goal is consolidation and penetration into the rock, acetone will evaporate and the Paraloid will create a barrier layer on or close to the surface preventing further consolidation. Constant application will prevent this from happening – you don’t need to reload the brush – simply squeeze a little or let gravity do the job.

These brushes are easy to clean and can be reused many times when looked after properly.

Immersion/Submersion (or a ‘Paraloid Bath’)

Soaking specimens in Paraloid can be a very effective way of guaranteeing penetration. However, not all specimens will cope well with immersion. Not all specimens can withstand immersion. Remember that Paraloid is only an effective consolidant once the solvent has evaporated - until then it is just like getting the specimen wet - and thus much more likely to break. For this reason, not all fossils are suited to this technique. The force exerted by the solution entering the pores at all possible points could be enough to shatter the specimen. 

For a Paraloid bath, a very very low viscosity solution is often advised (2-3%). Soak a specimen for a period of time. Support the specimen in a wire mesh where possible to help you when you come to take it out. When you take it out, popping the specimen on a sheet of cardboard or similar will allow you to remove any stuck to the outside quite easily with a little acetone. Alternatively, on a zig-zag folded sheet of metal so that there is maximum strength and support as well as minimum contact. If you put it straight down on a harder surface you may find that it sticks a little harder than you expected. Vacuum impregnation is also possible, but in the average home workshop this is a very expensive approach that may not always lead to success. 

Deciding whether submersion is the right way forward is a judgement call to be made on a case by case basis. 

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Perhaps one of the things that attracts and repels people in the same way is the reversability of Paraloid. One may assume that the bond isn’t strong if a reversible glue is used. On the contrary... you’d be shocked at how strong and even elastic Paraloid can be.

Paraloid goes first rubbery, then leathery and then hard and more brittle as the solvent evaporates. The fresher it is, the easier it is to remove. Brushing acetone onto a surface will begin to remove it, or alternatively the creation of a poultice with kitchen roll soaked in solvent contained in a polyethelene bag or sheet. This will take time, but at least it is possible. Alternatively heat the specimen close to the glass transition temperature of 40 degrees (if the specimen can take it).

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Do not use Paraloid in damp or wet conditions. Instead look into Primal Rhoplex WS-24. This is more akin to PVA. It is not as strong as Paraloid, but is an excellent field consolidant for wet conditions (e.g. ichthyosaurs on shale ledges or in rockpools).

Paraloid should be avoided if you need an extremely strong bond – provided by our cyanoacrylates or an epoxy adhesive. The same can be said if you need a very quick drying substance or join (superglue wins here), and conservation grade materials are not strictly necessary.

Paraloid has a high glass transition temperature, meaning that under normal environmental conditions under 40◦C it will not slump or budge. This does mean that it is not the answer to your problems when used as an adhesive in very hot environments – indeed, in Greece, archaeologists avoid using it as an adhesive for ceramics in the field, instead using it in cooler laboratories. This can be somewhat mitigated by using an acetone/ethanol mixture.