What is fossil preparation?
Fossils can be prepared or cleaned in a variety of ways depending on the nature of the fossil and the matrix (the rock) that contains it. All preparation methods exploit a difference between the matrix and the fossil, this difference can be chemical or physical. The most common method is mechanical preparation (as opposed to chemical) with either hand tools or with pneumatic (compressed air powered) tools. These methods exploit a difference in hardness between the fossil and the matrix or a line of weakness between the fossil and the matrix. Nowadays, most people use pneumatic tools such as air pens and air abrasives.
Click the link below to visit the shop, or keep reading to find out more about using these fossil preparation methods.
An introduction to fossil preparation tools Fossil prep air pens air scribes fossil preparation cleaning air abrasive mechanical preparation shop for sale
Fossil preparation air pens (otherwise known as air scribes, pneumatic hammers, air incisors, air engravers) are often the first pneumatic tool that the preparator will purchase and use.
They behave a little like miniature jackhammers which you hold in your hand like a pencil. Compressed air (generated by an air compressor) is used to push a stylus back and forth thousands or tens of thousands of times per minute. Different types and sizes are suited for different jobs, i.e. very large tools for where there is a lot of hard rock to remove, and very fine tools to remove fractions of a millimetre of rock at a time when working with fragile and delicate fossils.
An air pen is made up of a body (or handpiece), a stylus or nib (made from tungsten carbide), and a bushing which holds the stylus in place. The most powerful tools are impact-driven(which use a piston to hit the stylus), and the most delicate tools are pusher plate driven (where air is used to push the stylus). Click here for more information on how to choose an air pen to best suit the fossils you prep and the rocks that you encounter.
The idea is to remove the matrix without actually touching the fossil underneath with the tool, which is quite likely to damage it. The stylus, or nib, is used to chip away rock by holding the tool to the matrix and using small, regular scribing motions. It is important to let the tool to the work and use its own power to remove matrix, rather than being forced which may damage the tungsten carbide stylus (which is the material used because it is very hard – most importantly harder than the rock and resistant to the abrasive nature of the rock).
|A series of ZOIC air pens or air scribes, lined up from left to right in terms of power. You can see that the ZOIC T-Rex on the far left is considerably larger than the ZOIC Microraptor on the far right, which gives a good indication of what jobs each of these very different fossil preparation tools do.||Here's The Wobbly Fossiler (@the_wobbly_fossiler) working on a multiblock of ammonites in a limestone nodule. James is a professional fossil preparator (his job is to clean fossils!) who chooses to use his pens (pictured here with the ZOIC Chicago) in a blast cabinet. This is partly to keep his flat clean (he works indoors), and partly so that he doesn't inhale rock dust which can have devastating respiratory consequences if a high grade dust mask is not worn. The little lines on the rock are where James has been penning - he will smooth these down also using an air pen when he's finished.||This air pen (the ZOIC T-Rex) is fitted with a chisel stylus. This is to quickly remove the rock. This nodule is almost pure pyrite (it's called a cannonball nodule and the name is not misleading!). Pyrite is a famously hard material and the Yorkshire Coast rocks can therefore be quite difficult to prepare, at least quickly, and so a powerful air pen is a necessity.||This is a much smaller air pen in use, the ZOIC Microraptor, designed for fine detail work. Here it is being held at a high angle, almost perpendicular to the fossil, in order to work around and expose the ammonite without accidentally damaging the calcite. The air pen is being used to gently remove the remaining bits of shell, avoiding scratching the crystalline internal structure.|
Sometimes, the rock ‘pops’ away from the fossil. Other nodules or rocks are termed “sticky” by collectors; meaning the fossil and surrounding matrix are all strongly bonded together. This makes them harder to clean and often produces poorer results. Whilst it is possible to use an air pen to clean these fossils, it takes a great deal of patience and skill. This is where air abrasives come into their element. Often with ammonites, the calcite infill will be prepared and the original shell of the ammonite will be prepped away. When this calcite formed inside the ammonite’s shell it was often only very loosely attached to the smooth interior of the shell. This makes it possible to prepare the fossil by mechanically breaking away the shell of the ammonite to reveal an attractively coloured and translucent calcite fill. It often takes air abrasives to maintain the shell if this is the desired end result.
|Asteroceras obtusum prepped with an air pen. The shell has popped off to reveal the attractive honey-coloured calcite that has filled the chambers of the ammonite underneath. You can see the darker calcite is where the body chamber of the ammonite is. Photo, find and prep ©Chris Andrew||A very large example of Caenisites brooki, prepped with an air pen. The shell adheres to the ammonite better in these nodules and so is more often left on, but the rock is extremely sticky making it a very challenging prep job. There are a couple of areas of wear in which the shell could not be salvaged. Photo and prep © Chris Andrew|
Air Scribe for fossil prep
This video shows a particularly powerful air pen, the ZOIC T-Rex in use on a variety of British rocks. You can see the chips of rock flying away.
Now you know a little more about the purpose in air pens in fossil preparation, why not have a look at our guide to choosing an air scribe for fossil prep?
Air abrasive fossil preparation
Using an air abrasive tool in fossil preparation can be the key to the most successful preparation. This is the process of using an abrasive powder held in a stream of compressed air, being ‘blasted’ at the matrix. It is similar to sandblasting, but uses much finer particles. Many preparators use dental micro-blasters or air abrasive units which are essentially bench-top sandblasters for this job.
An air abrasive unit contains abrasive powder (more on this shortly), which is fed through a tube by compressed air to a handpiece which you hold like a pen. It’s a bit like using an airbrush for spray paint. The nozzle is held away from the rock allowing the powder to do its thing.
Air abrasives are usually used for finishing or really fiddly work and are most effective when the fossil is harder than the rock that encases it. The results when the right powder at the right air pressure is used can be astounding. Air abrasives can allow for a finish that wouldn’t be possible with an air pen alone, or allow you to do a job where vibrations of air scribing may damage the fossil.
There are a variety of different powders out there to put in an air abrasive unit. The most common in fossil preparation are dolomite, aluminium oxide, iron powder and sodium bicarbonate. These vary in effect as a result of the differences in grain size, grain shape and hardness. More information on choosing powders for air abrasion here.
|This is an air abrasive pen at work on an ammonite.|| This is a fossilised brittlestar, Ophioderma tenuibranchiata. By nature, these fossils are incredibly fragile and difficult to prepare. The legs twist, turn and overlap, all whilst covered up by the rock and so there is no telling how the limbs will be arranged. By slowly working along each leg using aluminium oxide as an abrasive powder at very low pressures it is possible to reveal the limbs one by one. Fossil found and prepared by Dave Halmkin. Photo credit ©Chris Andrew.
||This Arnioceras contained in a very sticky nodule has been prepared by The Wobbly Fossiler. He used an air pen to work down close to the fossil, carefully finding the top of each rib of the ammonite. He then used dolomite as an air abrasive powder to expose the rest of the ammonite. He has chosen to leave the beautiful waxy-looking shell on this example. Follow this link to find out more about the step-by-step cleaning of this fossil!
||This Walliserops trifurcatus has been prepared using air abrasives. You can see that it has been prepared in 3D. In this specimen, as virtually all others, some of the longer, more delicate spines have been broken off only to be prepped separately and then reattached. This 3D prep is extremely difficult using an air pen as the vibrations of even the most delicate can cause breakages. Using a very gentle air abrasive powder (such as sodium bicarbonate) allows for a super gentle approach.|
The angle at which the jet of abrasive powder is held is very important. A 45° angle can act to gouge, which is great when digging through matrix, but less good when working to the fossil. Low angles (25° or lower) are usually best when working on fossils as it allows for the powder to skim over the top of the fossil, rather than boring into it. The spray pattern can also be modified by holding the nozzle closer to or further away from the fossil - you can superconcentrate the stream or use it as a light all-over dusting. Keep the jet of abrasive moving rather than holding over a single part of the fossil for too long.
All professional workshops will contain an air abrasive unit (or many – each containing a different powder for a different purpose); and increasingly home set-ups are utilising air abrasives as good quality units are available at more affordable prices (such as the Vaniman Mobile Problast). The air consumption on air abrasives can be moderate to very high depending on the model, and so a more powerful compressor with a larger tank than would be used for air pens alone is required.
Here's a video of a crinoid fossil being prepared using a Vaniman Master Mobile Problast air abrasive unit with aluminium oxide powder:
Safety in Fossil Preparation
Fossil preparation can be a dangerous pastime if health and safety measures haven't been considered. Even if you're only working at home, taking a few precautions is essential. Inhalation of rock dust or air abrasive powders can be extremely damaging to your respiratory system. Please always wear a mask (minimum grade FFP3). Silicosis is a lung disease caused by breathing in silica particles (silica is a major component in sand and many rocks), which causes scarring of the lungs. Some fossiliferous limestones set underwater, and if you were to inhale that rock dust, you can imagine where it starts to set! When air abrading it is essential to work in a blast cabinet with suitable dust extraction (this also helps prevent an enormous mess - win win!). Wearing gloves is strongly advised as air abrasives can be skin irritants, or act to dry out the hands. At higher pressures, with a direct shot the abrasive flow can remove skin.