Preparing a Promicroceras ammonite from a limestone nodule- step-by-step fossil prep

In this post we’ll go over the very basics of preparing an ammonite in a nodule. This tutorial is designed to show you how to approach preparing an ammonite with a beginner’s set up (an air pen or air scribe, with no air abasive) - the same principles apply for those of you using an electric engraver, but it will take a little longer. The nodules from Lyme Regis and Charmouth are relatively hard limestone, and so most people use an air pen or air scribe to uncover the calcite ammonites within. Please make sure you’ve got eye and respiratory protection before you get going.

Our starting point, a Promicroceras ammonite in a nodule from Black Ven, between Lyme Regis and Charmouth in Dorset. Affectionately known by locals as a ‘Prom’ and the most common calcite ammonite along this section of the Jurassic Coast.

1. Fossil identification

It’s really important to have an idea of what you’re working with. OK, so it’s obviously an ammonite - this is a really important start! You know that it’s probably going to be round and spirally.

Ammonites come in all different shapes and sizes. We know that this one is from Black Ven, and so it’s not going to be a heteromorph as it is Jurassic in age. That certainly helps narrow down how we’re going to approach preparing it. It’s then a matter of genus identification if you can - some ammonites are spiny which means that you approach their preparation differently; some have very deep centres; some shallow; others have lots of ribs, others none at all. If you can’t identify your fossil initially, try and work it out as you move along.

2. Plan your prep

One of the hallmarks of a professional preparator, is not necessarily the prep of the fossil itself, but the way the matrix around it looks. Every mark you make with an air pen is there to stay, so think hard where the ammonite is and how you want the matrix to look at the end.

You’ll also want to think about the angle at which you approach the ammonite. You’re less likely to damage it if you approach it from further away and at a lower angle, but you may end up marking more matrix than you want to.

It often helps to draw on the rock and mark out where you want to remove rock, or alternatively where the ammonite coils may lie (in a spiral shape. Think about the thickness of the rock and how much you have to remove, and how this will affect the angles at which you can approach the ammonite. 

 3. Grab your tools

This tutorial is on using air pens (or air scribes) to expose an ammonite, however many of the principles apply to those of you using and engraver. We’re using ZOIC PalaeoTech tools; the ZPT-TR T-Rex and the ZPT-MR Microcroraptor models - The T-Rex is very powerful and will help us get close to the fossil and force transmitted to the rock may just 'pop' the rock away. The Microraptor is very delicate with a very small tungsten carbide nib which is perfect for detailed work.

 4. Starting working on exposing the outer whorl of the ammonite

Always start off by removing the rock from around the ammonite. Here, I’m stopping my stroke 2-3mm away from the fossil itself just so that I don’t damage it. The whole trick with this is not to touch the fossil with the nib, not even once!

I’m using the ZPT-TR to quickly remove matrix from around the fossil. I’m working from the line I drew, and using the tool at a reasonably low angle to gently remove a little rock at a time. The important thing to bear in mind is to turn the rock around so that every stroke is approximately pointing towards the centre of the ammonite. 

Some fossils ‘pop’ better than others. When this doesn’t happen, the fossil is referred to as ‘sticky’. These are much harder to prepare as you’re more likely to damage the fossil as you have to get that much closer. It can be done but commercial preparators will usually use air abrasives as opposed to air pens for these jobs, or just chuck certain nodules back on the beach because they know they can be troublesome and time consuming. Luckily for us, this fossil isn’t very sticky.As you can see, in this photograph, a piece of the matrix has ‘popped’ away from the ammonite. Fossil preparation is usually exploiting the different properties of the fossil and the rock, and the resulting plane of weakness between them. In this case, the plane of weakness exists between the ammonite’s shell (on the bit that popped off) and the calcite infill. The aim here is to expose the calcite infill by removing the rock and the shell.

The pen marks can be seen here (highlighted in black). I keep turning the fossil round to make sure I’m always approximately approaching the centre of the ammonite. The tool marks are roughly in a circle around the ammonite (so I don’t damage the matrix too far away from the fossil), and they are not going within 2-3mm of the ammonite. Therefore, I am much less likely to damage the ammonite.

Now we’re that much closer to the ammonite, I can switch over to using the ZPT-MR, designed for fine detail work. I can now use the tool at a low angle, whilst turning the rock and approaching the ammonite and stopping just before (within fractions of a millimetre) I hit the calcite of the ammonite. The rock is more likely to vaporise than ‘pop’ with this sort of tool (which is pusher plate driven), as the beats per minute are that much higher. This allows more control. If the rock has already ‘popped’ this will always provide a better finish, but where it hasn’t you’re better off using a finer tool where you have more control to avoid so called ‘dink marks’.

5. Preparing the Inner Whorls

As you can see, I’ve worked around the ammonite and carefully removed the matrix and exposed the outer whorl.

Now it’s time to approach the centre of the ammonite. Always work from outside to inside. Work from the outside whorl and keep turning the ammonite round until you reach the centre of the spiral. If the centre of the ammonite is covered in rock, you don’t want to be relying on guesswork as to where the whorls are. 

Keep the pen at a high angle as you do this; almost perpendicular to the ammonite. You want to avoid the side of the nib hitting any of the calcite and damaging it. By working with the tool at a high angle, you have much greater control over where the nib goes.

In this example, there are a few bits of shell which are overlying the calcite which can be removed with an air pen. Be very careful not to touch the calcite, as it will leave a hole! If a chamber inside the ammonite is hollow you will punch through it. 

You’ll notice as you go round that there will be a white line following the spiral (see above). This is just where you have penned the shell of the ammonite and the calcite has broken. This is totally normal and unavoidable. I’ll show you how to get rid of it later! You can also see a white dink mark when my hand slipped a little (circled in orange).

6. Smoothing the Matrix

The white line of broken calcite is also very clear in this shot. Here I have begun smoothing the matrix using the ZPT-TR. By gently scrawling on the surface with the nib perpendicular to the rock, I was able to quickly flatten out any lumps and ridges, and also trace a circle in the rock to ‘frame’ the ammonite. All the little dots you can see are due to the fact that the tool runs at low pressures with only a few 1000 beats per minute.

Every preparator has a different technique for final finishing work, and as I said this comes largely down to the matrix. You will notice that many preparators have a ‘signature style’, and you’ll certainly find yours with time and practise. Some will use air abrasives, some will use a Dremel, and many will use side to side sweeping motions with their air pen. This is inadvisable and will damage any air pen over time by wearing away the bushing and put strain on the tungsten carbide nib. We've hardened and tempered the bushings of all of our tools, but eventually you could end up with a nib that wobbles about which makes it really hard to be accurate. We recommend using a high pressure tool to very lightly draw on the surface at a high angle, almost as if you’re colouring in! Neaten up the edges of your etched frame.

7. Finishing Touches

Now it’s finally time for the real finishing touch. With these calcite ammonites, a light coat of varnish will really set off your work. We use our Clear Gloss Varnish for Fossils as it gets rid of all the dink marks, and that white spiral where the calcite shell has been broken. We always make sure to paint it on carefully (without getting on the matrix), and then quickly dab off as much as possible with a piece of kitchen roll to remove the artificial shininess. It always looks nicer when all you do is restore the natural sheen of the fossil. Try to use a firm dabbing motion with the kitchen roll, so that you don’t spread varnish on the matrix. You can do a little twist in the centre if you need to.

8. Ready for Display

Voila! The ammonite is fully prepared and ready for display on your mantelpiece. Despite being common fossils, ammonites can be fiendish to prepare, so don’t beat yourself up if it’s not going well! With plenty of practice, you’ll find yourself improving. My greatest advice is to take it slow, and use some form of magnification.