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Onchopristis tooth 40-50mm
Preparation is the further extraction of a fossil when it is completely or partially embedded in the rock (also called matrix). The tools and techniques are highly dependent on the properties of the fossil and the surrounding matrix. The preparation of a fossil is seldom easy. In addition to customized tools, it often takes some time and patience to prepare a fossil. Preparation is often not without risk: the wrong approach can lead to damage or destruction of the fossil. However, proper preparation can show details that give special value the fossil.
Example of different steps in the preparation of a trilobite
The knowledge and mastery of preparation techniques is a discipline in itself, which often requires a lot of practice. Although you can find out about basic techniques both online and in books, much of your personal experience will be based on trying, experimentation, and trial and error. This may start with a lot of error, especially in the beginning, so it is a good reflex to always take fragmentary specimens from the field to try your preparation techniques.
A good preparation always begins with a thorough study of the fossil. Pay attention mainly on the following issues:
- What is the fossil and how does it extend in the matrix? Often you have to anticipate where the fossil is in the matrix during preparation. Otherwise you run the risk that you cause scratches or damage. Try as much as possible to identify the fossil and try to understand the anatomy of the specimen. Take into account that fossils may be deformed by geological processes.
- How stable is the fossil, are there cracks visible? Some preparation techniques cause vibrations that cause fragile specimens to burst, or cause existing cracks to increase. Sometimes stabilization is required before the preparation is started. This can be done by applying a very low viscosity glue solution into the cracks. Preferebly a fully reversible glue like Paraloid B72.
- Are there other fossils present in the matrix? Sometimes there are several fossils in the same block. If you focus too much on a fossil, you run the risk that you will missany other fossils in the matrix and damage them.
Then you will have to decide how you approach the fossil. Wether you want to get it completely out of the rock or you leave a part of the matrix is a choice to make. The latter can be very beautiful, and it is sometimes useful or even necessary for the stability of the fossil. In case of possibly scientifically important fossils, it is advisable to always leave some matrix on the fossil. This matrix may contain essential information (e.g. microfossils), which may be required for further study and dating of the specimen.
Finally, you choose what tools and techniques you will use for the preparation. Every fossil is different and the technical possibilities are endless, but below we still try to explain some common basic techniques and tools.
Essential for proper preparation is patience and concentration. Most mistakes happen by rushing to proceed or to put too much pressure on the matrix. The preparation of some specimen may require dozens, in extreme cases, even hundreds of hours of work.
With a manual preparation we mean a preparation by hand, without the aid of electric or compressed air driven tools. It is a technique often used in softer rocks, or when fragile fossils and fine detail should be exposed. In the latter case, a manual preparation can take a very long time. Manual preparation can be done using a variety of tools. Heavy-duty, one often uses a small chisel and light hammer, the finest work is done with extremely sharp blades and needles under the binocular. Keep in mind that the use of a hammer and chisel causes vibration, which is not wise with fragile fossils. For controlled breaking of pieces of stone, pliers can be used.
Manual preparation using a hand tool.
Compressed Air System
A compressed air system is the core of a preparation workshop. A basic configuration consists of at least one compressor, pressure regulator, dehumidifier, tubes and connections.
With a simple compressor you go a long way. If you must work in the same room as the compressor is located, and / or if there is a risk of too much noise for family members or neighbors, you can opt for a low-noise (whisper) compressor. These are a lot more expensive. Compressors that are equipped with a pressure tank have the advantage that they have to operate less often. The compressor should be able to deliver about 8 bar (about 116 psi) to the set for an efficient compressed air system for preparation.
A pressure regulator is usually built into the compressor. Additionally, you can choose to mount a pressure regulator at your workplace. This is best combined with a dehumidifier, it is always important that you can work with dry air. If necessary, dehumidifiers can be connected in series, but the general rule is that you build a system as simple as possible. You can connect different tools on the same system. When tools need different pressures, it may be needed to install multiple pressure regulators, so you can switch between them.
Sometimes, it is recommended that a lubricator must be interconnected to lubricate the tools. Your tools after the dehumidifiers We suggest rather not. First and foremost because continuous lubrication is not strictly necessary, and this oil gets on the fossil. There are also specialized pressure pins for fossil preparation that do not need be lubricated. Finally, the oil is bad for sandblasters. You can lubricate individual tools by regularly putting a regular drop of oil in the pipe and just blow through it before continuing working with it (check with a paper towel or oil is purged). You make best use of fast connectors for your tools.
With all above in mind, a compressed air system could look like this:
- pressure regulator with manometer
- air chisels
- adding oil
Preparation with compressed air chisels or engraving tools
Preparation needles and chisels operated by compressed air are an almost indispensable tool for the amateur paleontologist. The preparation needle is connected to the compressor with a hose and vibrates to tens of thousands of times per minute. This type of tool allows high precision to remove stone. Originally derived from classical engraving tool, there are now several distributors on the market that developed preparation needles specifically for fossil preparation. Although they usually cost more than traditional engraving needles, they are often superior for preparation.
Example of a pneumatic chisel
The use of compressed air has the effect that the preparation tool itself hardly vibrates, which allows precise work and little tiring of the muscles in the hand. This is in contrast to electric-powered tools were you hold the tool with the electric motor in the hand. This works little ergonomic and makes delicate work impossible. The big price advantage that makes electric engraving tool so attractive, does not outweigh these disadvantages. We can strongly advise against this kind of equipment. The fine preparation work is typically done under magnification, ideally a good stereo microscope with a long suspension and a sufficiently large working distance.
Preparation with an air chisel
Preparation with sandblasting tool
In some cases, the technique of micro-sandblasting can be used in the preparation of fossils. This is a very complex technology, where you spray very small grains (the media or abrasive) on the stone at high pressure, making it wear. You can compare it with an artificial and accelerated erosion process. Especially when the fossil is relatively harder than the surrounding matrix, this technique can be interesting and effective, and deliver outstanding results in some cases. However, if misused, a sandblaster can also destroy a fossil in seconds.
There are different types of blasters, and prices can vary widely. But usually the price is a good indicator of what you can do with a sandblaster and how exactly you can work with it. Roughly classified you have sandblasters that work with a container under pressure, and sandblasters that work with the aid of a venturi-system (in which a container is put in under-pressure). Venturi systems are relatively simple and thus often the most economical. More expensive systems may include the following advantages: finer work, less blockages, more containers for quick selection between different media, start and stop quickly, independently adjustable flow pressure, and the like.
All sandblasters are basically at least one container with blast medium, a pressure regulator (often with built-in dehumidifier), a foot switch for operation, and a jet pen with (replaceable) nozzle. This is connected to a compressor. Please note that sandblasting often requires more of a compressor than the normal preparation needles. The use should be done always and without exception in a locked cabinet in under- pressure as the fine dust that is released when blasting is bad for the lungs and, depending on the blasting medium, potentially toxic. A mask does not provide sufficient protection here. Some types blasters are sold with cabinets, but you can also build your own relatively easy. Also, the suction system that should put the cabinet in under-pressure should perform well. A simple vacuum cleaner is not sufficient, an industrial vacuum cleaner with i.e. additional nylon filter is a minimum. For very fine or toxic products you may need to invest in a better system.
Example of a home made blast cabinet.
Sandblasting as a technique is very complex because there are many variables that each have a specific influence on the result. The main ones are:
- The blasting media: both the hardness, grain size, angularity and purity affect how quickly the matrix of the fossil are eaten away. Of course, the hardness is a very determining factor.
- The pressure under which work is done can be very decisive for the speed at which the matrix is eliminated. For many devices, the pressure also determines the amount of blasting medium that leaves the nozzle per second.
- The flow rate at which the medium exits the nozzle has a direct influence on the rate at which the matrix is eliminated. Often this can not be adjusted separately, and, depends on the set pressure.
- Properties of the sand blaster unit: The grain size of the medium that can be used is heavily influenced by the sandblasting tool, and the nozzle determines how precise you can work.
- Technique and skill of the preparator are very important, since the technique is capable to blast a fossil away completely in seconds. Selection of blasting medium, pressure, etc. is partly a matter of trial and error: so remember to collect test pieces.
Water is a great enemy to sandblasting, because even the slightest bit of moisture can cause blockages. Therefore, the incoming air must literally powder dry. Provide the necessary dehumidifiers in your pipe circuit, avoid temperature differences over your hoses and place your blaster in a dry place. Best put a sachet of silica gel stab in the drums with a medium or if necessary a bag of rice. Some media can be reused after fine sieving, others can be used only once.
Commonly used blast media for fossil preparation include iron powder, dolomite, sodium bicarbonate and aluminium oxide, of different grain sizes. Sometimes also mixtures of blasting media are used. Because it requires a lot of effort to work out the "right" blasting technique for a particular fossil of a given site, you will hear of experiences from other preparators usually only discrete or very little.
The fine sandblasting typically is done under the stereomicroscope, with a very fine abrasive medium and a fine nozzle.
With chemical preparation, the matrix is ??removed by using chemicals (agents). Here, too, there is a wide range of techniques are available, which must be carefully selected on the basis of the properties of matrix and fossil. The idea here is that the chemicals erode the matrix, but not (or much slower) the fossil. The technique is not applicable to internal molds, the fossil itself should be preserved. Misapplication of chemical preparation techniques can have disastrous consequences, even here, so the advice to test new techniques first on test pieces.
The process can vary from the complete immersion of the fossil and rinsing with a neutralizing agent, which may be repeated in cycles, up to the application of the agent on small parts. By immersing fossils in a reactive substance, first check whether the fossil is free of cracks and fractures. Along cracks the agent can indeed quickly penetrate beneath the shell and work from the inside. If the fossil has similar chemical properties than the matrix, a protective layer may need to be applied to the parts that have been uncovered. For example, in repeated cycles becoming a bit more of the fossil to be uncovered and be provided with a protective layer. After chemical treatment, the fossil should be neutralized each time, in order to prevent the remnants of the agent to continue working.
Without claiming to be exhaustive, some techniques are briefly discussed here that are also accessible to the amature collector:
- Working with acids: a mild (and safe) acid is vinegar. Allows you dissolve calcium slowly. However, fossils in limestone matrix often consist of lime also, and are therefore also affected.
- Working with bases: a base commonly used in chemical preparation is potassium hydroxide (KOH), available in flakes, which are applied locally, and then moistened. Let stand, neutralize with vinegar and rinse. KOH is a very corrosive substance so take the necessary precautions. KOH solution is quite viscous, it feels very greasy. Generally, chemicals that have a greasy feeling (concentrated sulfuric acid, bases) very dificult to rinse off with water. KOH sometimes penetrates very well into a fossil and its matrix. Neutralization will often be only at the surface and then a long and rinse well with water is unsatisfactory in some cases. So, watch out for important specimens, treatment with KOH is not without risks, there are cases where the preparation seems successful, then the fossil falls apart after 10 years or more after the preparation.
- Rewoquat: this is a specific surface reactive substance which is very popular for cleaning fossils in a (calcareous) marl matrix. The fossils are immersed for some time in this agent, after which they can be rinsed with water, and then neutralized. Rewoquat can be easily recycled. It is a quaternary ammonium substance, which finds its application within the so-called "fabric softeners". The operation is very similar to that of an emulsifier. It binds to particles and makes this "water-soluble". The Rewoquat does not solve the matrix, but make sure you can rinse of part of the matrix with water. Rewoquat is popular for the cleaning of sea urchins which always contain an opening, or 2. It is advisable to cover the openings up with a layer of glue so that the Rewoquat does not penetrate in the matrix inside the fossil. This in order to prevent long-term damage from the inside out.
Other preparation techniques and tools
Ultrasonic cleaning: an ultrasonic cleaner is a container in which the fossil is submerged in a detergent (water, with or without an added detergent), then the unit sends ultrasonic waves through the water. As a result, small particles are loosened. This technique is primarily applicable to weak adhesive matrix.
Grinding: grinders in various formats can an important tool in the preparation. For fine work you can choose a diamond cutter on a rotating device (type dremel). Greater table cutting blade can be useful for grinding matrix at a specific size. Watch your fingers, and wear appropriate protection against fine dust and rock fragments (masks and goggles). With the help of a grinder you can grind ofa large amount of matrix of a block rapidly. Make a checkerboard pattern with the saw. Each block can then be individually chopped away.
Polishing: often used for decorative purposes and in order to make, fossils tradable but in some cases polishing can well be justified, in particular, in order to make internal structures visible. As cephalopods can be cut and polished to make the rooms, and siphon visible. Some of the fossils have been described on the basis of the internal structure and can only be identified accordingly (e.g. certain corals). Sawed and polished pieces can also have an educational function.
Freeze-thaw: a much-discussed technique but rarely produces good results. The idea is to mimic natural frost weathering. The fossil is immersed in water for a while and then put in the refrigerator wet. The water freezing in small cracks and crevices expands, and the stone can crack. The hope is that this happens at the height of the transition between matrix and fossil. This process schould be repeated multiple times before result.
For fine preparation are optical devices often indispensable. Those who regularly prepare fossils, it is best to invest in quite a decent binocular stereo microscope. Budget friendly alternatives are table magnifiers on a movable arm or loupe glasses. Microscope with transmitted light are not suitable to view most fossils (with the exception of thin sections).
If you are going to invest in a microscope for preparation, there are a number of issues that are of specific concern to your choice:
Binocular and stereo: a stereo microscope has two eyepieces which deliver a separate image. This gives you depth perception. This is essential for preparation.
Magnification: usually you only need a relatively small magnification. A usable magnification is 10x, but many microscopes offer a number of magnifications, either incrementally by replacing eyepieces, or continuously using a zoom button, which works a lot more versatile. Magnifications between roughly 3x and 40x can be useful, a zoom range between 5x and 20x is usually more than enough, even for versatile use.
Working distance: the working distance is the distance between the bottom of the microscope and the point where focus is, and where you work. It is very important that this distance is large enough to accommodate your hand and preparation tool. A guideline is 10 to 15 cm or more. If the microscope offers a smaller working distance than 15cm, then test in advance how that works with your tools.
Suspension: a microscope for preparation is suspended from a swing arm, so that there is sufficient space to put the fossil under the microscope.
Lighting: trust that you are going to need a lot of light, especially at higher magnifications, and that this light best should not be too hot. A classic but usually expensive solution is to work with a cold light source, wherein the light is directed with the aid of one or more flexible fiber-optic cables. Please note that these light sources in the long term are not always dust resistant. Nowadays, there are more price efficient alternatives that make use of LED technology.
Image quality: good image quality is less tiring to the eyes, but often comes with considerable additional cost. This is a classic trade-off, but not least important, since you look through the microscope during the preparation sometimes for hours. Try to choose WF or "wide field" eyepieces. These give a broader "exit pupil", which also works a lot more comfortable.
Photography: Some models have a third eyepiece for a camera, or the opportunity to attach a camera to one of the eyepieces with an adapter.
To prepare fine details such as this compound eye you have to work under the microscope. One lens is damaged by a hit with the preparation needle.
Thanks to Ivo Kesselaer for his additions to the text.
Translation by Herman Zevenberg (webmaster).
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