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. Historically, people used ordinary pneumatic engravers, but these have been superseded by modern, purpose-built tooling.
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 PalaeoTech 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
Want to see an air pen in action?
Now you know a little more about what an air pen does, why not read our guide to choosing an air pen for fossil prepping?