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.

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.

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.

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). 

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 and will remain in solution. 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.

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.

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.

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.

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 apart as soon as it dries out?

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.

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

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 flexibility. The best way to use it as a glue, is to use a very 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.

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 contrast. The more porous the rock, the less the sheen.

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.

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. 

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. 

Preservation of pyrite is always a contentious topic, with a lot of misinformation flying around. They even used to believe that it was a bacterial problem that could be treated with antibacterial ointment. Pyrite oxidises in contact with moisture and oxygen, which is why you may find that some of your finds have crumbled to dust. There are various treatments you can do to help prevent this, including using a vacuum chamber, ammonia gas or Pyrite Stop (ethanolamine thioglycollate). After one of these steps has been taken, Paraloid can help preserve your finds by sealing the specimen off from the air as best as possible as it is oxygen and moisture that cause the reaction leading to pyrite decay. 

However, it's actually a different sort of Paraloid that will work best as a protective barrier when working with Pyritic Fossils. Paraloid B-67 is an isobutyl methacrylate polymer and so is of a different composition. It is hydrophobic, providing excellent water resistance (a suitable sealant following Pyrite Decay treatment in Pyrite Stop). It can provide an excellent barrier against both atmospheric moisture and gas (oxygen); with much lower permeability than Paraloid B-72. Storage in low humidity environments remains essential, but Paraloid B-67 can provide an extra barrier layer between atmosphere and fossil. 

The only thing to really mention is that if the specimen has been rinsed in water (but not soaked) to remove salts, wait for it to dry fully before applying Paraloid. If choosing immersion (not always appropriate), 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, soaking in acetone first.

For more advice on how to prevent, treat and store pyritic fossils read our article linked here. 

Damage will occur to bones and teeth even in a buffered solution of acid during chemical preparation. Bones and teeth can become extremely brittle and fragile, and even completely disintegrate. This is partly because bones and teeth are porous and therefore not entirely composed of one mineral – they will often have calcite (calcium carbonate) infill where blood vessels, pores and marrow once were. This is most prevalent in cancellous (spongy) bone. It can also damage surface detail and cause a huge loss of strength. The solubility of calcium phosphate is much higher in formic acid than acetic.  

Bone can be protected with a 25-30% or more coat of Paraloid B-72, although this does need reapplying every cycle. It can also be protected by wax or plasticine. Increasingly, Synocryl 9122x is being used as it does not require constant reapplication and provides and excellent resistive barrier. 

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).

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.