Search the Community

Latex is very useful in fossil preparation and photography since it allows for extremely detailed casts to be made without damaging the original specimen. They are often whitened for photography using ammonium chloride or magnesium oxide, resulting in beautiful contrast of even the tiniest details. I have been experimenting with latex for a while now and have encountered a few issues that have taken some experimentation to fix, but now I am achieving results I’m happy with! Here I will discuss these issues and their solutions, as well as my method for creating the casts. Ammonium chloride and photography is another topic which could be discussed elsewhere. When is latex useful? Latex is most useful when casting a specimen where only an impression remains. This is often the case for fossils where the original material (often calcium carbonate or calcium phosphate) has been leached out, leaving only a cavity. Some specimens may not have a perfectly clean cavity and require some cleaning which is the case for some of the Devonian fish material I have worked on. The bones are split through the middle and are “rotten”, so it is too difficult to prepare them mechanically. To prepare them, the rotten bone is dissolved away in hydrochloric acid which dissolves only the calcium phosphate making up the bone. This leaves the matrix untouched and creates a perfect mould ready for latex. This can be done with most fossils in siliclastic rocks since the matrix won’t react with the acid, but *cannot* be done with calcareous matrixes since the acid will dissolve the fossil and matrix indiscriminately! Latex is also useful in creating replicas of fossils. In this case, latex is applied using the same method discussed later except with a positive instead of a negative. This latex mould is then used to cast the fossil in whichever desired material (eg resin or plaster). It is also worth noting that latex should not be used on some subjects since it can damage or ruin them. This is the case for fragile specimens with thin cavities or overhangs, the latex can rip them right off! Latex also shouldn’t be used on very porous matrixes (such as diatomite) since the latex will rip the surface right off the rock if allowed to penetrate too deep. Photo of a Devonian fish scale prepared using the technique mentioned: My original methods and their issues Originally, I applied latex straight out of the tub onto the rock by stippling it on. The first layer was always thin, so this process was repeated until a thick layer was built up and could be peeled off. This method works well for flat, featureless subjects but whenever a subject with fine details was used (such as bumpy placoderm armour or trilobite segments) it was impossible to avoid bubbles. Bubbles in the first layer are a huge issue since they can obscure detail and are unsightly. Some papers mention first wetting the specimen and then applying the latex. This was better, but I found that the latex would dry unevenly and sometimes not reach the specimen due to the water and latex not mixing and resulted in reduced detail. Another failed solution was watering down the latex for the first few layers. This method was much better, but still resulted in too many bubbles. Most of the bubbles were formed during the application when trying to force the latex into small cavities, so there had to be a way to allow the latex to evenly flow straight into every cavity and hole. The solution? Dish soap! This method combines both of the previous methods but uses dish soap in both the water on the specimen and the watered down latex. Once the specimen is wet and the deep puddles blown or drained off, the latex is applied to the specimen with a pipette. Water is capable of conforming to the specimen perfectly, and the addition of dish soap allows the latex to follow anywhere the water does. This method is excellent since it eliminates most bubbles from forming, most of the bubbles form when trying to cram latex into the specimen with a brush. My current method Firstly, I start by washing off any dust or acid preparation residue from the specimen. If dust or dirt is covering the specimen, it will obscure fine details that become obvious when whitened. Here I am using a small crinoid impression where the fossil has been naturally etched. Next, mix up some latex. The ratio isn’t very important, I usually add a dollop of latex into a small container and splash a bit of water onto it to water it down. I then add a drop of black Indian ink to blacken the latex. Finally, I add a drop of generic dish soap into the latex and mix thoroughly. Using a spray bottle, I then wet the surface of the specimen with dish soap mixed with water to ensure that every fine detail of the specimen is wet. Once sufficiently wet down, excess water can be blown or drained off. Now for the first layer of latex! I use a pipette to drop latex onto the specimen, and it can immediately be seen that the latex mixes evenly into the water and fills all of the cavities without any effort. Sometimes the latex needs to be brushed in to the water for it to mix evenly, and in this case the bubbles can be washed off with more latex by holding the specimen over the container and dropping fresh latex onto it. Then the latex can be blown with a blower to spread out puddles and pop stubborn bubbles. It is important to note that any deep fissures *must* be drained of excess latex for the first few coats. Since the latex is watered down, it dries in a downward fashion and becomes a thin membrane over the specimen. If this membrane forms at the top of a narrow fissure, the latex inside will not dry and/or form cavities from the evaporating water. To avoid this, blow or drain excess latex out of the cavity and allow it to dry on an angle or upside down to prevent further pooling. Blowing the latex to thin out puddles is also useful as it can pop any bubbles which do end up forming (I use a camera cleaning blower). Once this first layer has dried completely, it can be coated multiple more times until the detail of the specimen is obscured by latex. Layers of incrementally thicker latex (less watered down) can also be used. Here is a photo of the first coat, and then the nearly dried first coat of a different specimen. This shows how thin the first few coats can become! A photo of what happens when deep fissures aren't drained properly, resulting in air pockets: Once there is a thick enough layer of latex a final unwatered down coat of latex can be applied. Since all of the detail of the specimen has already been filled, bubbles are perfectly fine. After one or more of these thicker coats, a backing can be applied. I tend to use cotton wool or chux, but any absorbant fabric will work. Cotton wool is particularly useful on specimens with large cavities or sharp edges since it provides a thick backing and fills larger gaps than flat fabrics. I apply it by first brushing on a thick layer of latex, then laying the cotton over it and packing it into the latex with a brush. It is important that the latex saturates all of the cotton so that it is adequately strong and doesn’t rip during removal. A photo of the backing I chose for this specimen. I think it is a product used for sealing, but it works just like chux. A fresh peel: The same latex photographed with ammonium chloride: Hopefully this is of use to somebody! The method outlined certainly isn't the only way to successfully latex fossils, but I find it to be the best for capturing fine detail which would otherwise be difficult. Would love to hear the experiences of others well.

I am having trouble coating this Isotelus plaster cast. This is my first time attempting the ammonium chloride coating method. First I painted the cast with a thin layer of black acrylic (in retrospect, I probably should have used watercolor?). Then made a number of tries to coat the cast with ammonium chloride and was unsuccessful. I tried placing the cast above the smoke, below and directly in front of the flask, nothing worked. I'm wondering if humidity could be an issue? Has anybody tried applying ammonium chloride as a fine liquid spray or magnesium ribbon? Any tips appreciated.

Whitening of fossils with ammonium chloride is a common and effective method to enhance the contrast for pictures. Here you can download a short "how to do video" : https://www.researchgate.net/publication/287444709_Whitening_fossils_with_ammonium_chloride Ammonium chloride is water soluble - the process is reversible (if the fossil tolerates rinsing with water). Have fun Thomas