Polishing can be a the perfect way to enhance the beauty of a fossil. It is an aesthetic approach to fossil preparation, usually performed on fossils which either are found so abundantly that their scientific value is limited, or fossils that just 'look best' when they're polished. Sometimes they are used in arts, crafts and jewellery in their polished form. The art of grinding, cutting, shaping and polishing rocks, fossils and minerals is known as lapidary. Polishing is suited to hard, impermeable stones and minerals like limestones, calcite and pyrite. It doesn't work on soft, porous rocks like shale or chalk.
Sometimes the isolated fossil is cut with a trim saw and polished, sometimes it is left whole and polished in its original shape, and sometimes the fossil and its surrounding matrix are polished (e.g. fossils in nodules, amber with fossil inclusions). Polishing fossils is something you can do at home. There are machines that make it faster and even semi-automated systems in lapidary workshops, but all you need to give it a go at home are some unpolished fossils and some wet-and-dry paper.
Virtually any hard fossil in hard, impermeable rock can be polished, but the greatest successes are with crystalline fossils or ones that otherwise don't show features particularly well. For instance, a very pyritic lump of ichthyosaur bone may be impossible to otherwise prep, with the only way to enjoy it being by cutting and polishing it. Beds of scattered shells or crinoid ossicles are other popular candidates, as is some petrified wood. With ammolite fossils from Canada (ammonite shells are preserved as ammolite, an opal-like rainbow coloured gemstone), the amazing colours are only brought out in the polishing process. Polishing can turn an average rock or poor fossil into a really beautiful piece. Petoskey stones are fossilised coral pebbles that take a polish really well, and you will have all seen the ubiquitous cut and polished Madagascan ammonites and Moroccan Orthocerasfor sale in the shops.
Although cutting and polishing is very much an aesthetic (and often commercial) approach to fossil preparation, interestingly some scientific discoveries have been made because a fossil has been sliced and/or polished. Bear in mind, unless you are explicitly looking to reveal the internal structure of a fossil, polishing will usually remove features that identify the fossil to a genus or species level. Polishing fossils can be a polarising topic within the fossil community and opinions vary dramatically depending on the type of fossil and locality. We would definitely recommend making sure that the fossil you are working on doesn't have a scientific value before deciding to slice or polish it.
This ammonite from Charmouth has been ground and polished to reveal the chambers inside, and allow the light to shine through! This ammonite was a good candidate because it was overgrown with iron pyrite, which would have made it a very difficult traditional prep job. Pyrite polishes up particularly well, but always make sure you're grinding wet rather than dry as the dust is dangerous. Photo: Lizzie Hingley of Stonebarrow Fossils.
Cutting Fossils for Polishing
Of course, not all fossils need to be cut to be polished, but some do. Fossil cutting can be performed on a trim saw or slab saw depending on size. Some people use cheap tile cutters, which works for smaller pieces, but the blades are very thick and so you end with a lot more wasted material.
Fossils can be carefully sliced using specialist equipment. You will have seen the famous Madagascan polished ammonites for sale, but there are plenty of others that cut and polish beautifully found all over the world. This technique is particularly popular with ammonites, where you can expose the internal chambers that in life would have filled with gas or water to allow the animal to orient itself and rise or fall in the water column. This is particularly effective when the chambers are filled with calcite crystals. If you would like your fossils cut so that you can polish them yourself, we recommend consulting a professional for this service, and you can find one by looking at our Fossil Preparation Services Business Directory.
Slicing fossils is skilled work. Getting the exact centre of the ammonite is difficult and takes a thin blade and a great deal of experience. A millimetre in the wrong direction and you've missed it - or it may not have been present in the first place. With smaller ammonites, often only one half will be kept and the other discarded as the thickness of the diamond cutting blade is thicker than the centre of the ammonite. Larger or thicker ammonites may be able to be kept as matched pairs.
Single ammonites can be cut, as can entire nodules or slabs. The most famous of these slabs is the Marston Marble from the UK; a rare Jurassic stone packed with ammonites. This was used in Victorian times in the village of Marston Magna to make tables, tombstones and more. It is and was often cut and polished, but increasingly it is being mechanically prepped (an enormously time consuming process) as its rarity increases (it was discovered by sinking a well and so the layer is buried deep underground).
Cut and polished block of the famous Marston Marble from Somerset, UK. Marston Marble is rare, as it is no longer quarried. It was used in Victorian times to make tables, tombstones and more. The orange coloured cracks you see are calcite - as this is a septarian nodule. With thanks to Summers Place Auctions for the image.
Professional lapidary equipment set up for stone cutting and polishing. Commonly used for the polishing of minerals and gemstones. Photos: Thomas Farley.
Polishing Fossils at Home
1. Setting up the Workspace
Make sure that you've got a few supplies handy! You may wish to cover your table with something to prevent water damage or damage from the grit. You will need some good quality wet-and-dry paper in 80 grit, 220 grit, 400 grit, 800 grit and 1200 grit (written on the back); a tub of water; tin oxide polishing powder; rotary Dremel-type tool with felt polishing tips; bar of soap (optional); double sided tape (optional); and a water filled garden mister bottle (optional). You may wish to wear gloves, and if you don't please be careful of your fingertips. You might want to lay down some newspapers, and have some old towels around to dry off the fossil and table. And a fossil that needs polishing!
Set up your workspace. It's messy and wet, so make sure your table is covered if it's not waterproof. There will probably also be spatter, so if the weather is suitable it may be worth working outside. You can tape your wet-and-dry paper to the table if you have a flat surface to polish, or if you don't want to hold it to the contours of the fossil (for non-flat surfaces). If you want to hold the wet-and-dry paper in your hand, make sure it's flexible enough to curve around the fossil, but not so thin that it rips or tears. The wet-and-dry paper we stock has been carefully selected due to its durability and flexibility. If you don't need to use the whole sheet of wet-and-dry, you can cut it up, but make sure each sheet has the number written on the back so you know which is which.
2. 80-Grit roughing work
If there are any ridges or very rough patches to work down, use the 80 grit. Feel with your fingers for any bumps or dips which may have been created when the fossil was cut - you want to work the fossil down until it's a flat surface and contacts the paper at all points. You may be able to start with a finer grade if your fossil is sliced (cut) without any particular features, ridges or scratches. But if you're polishing a natural stone, nodule or fossil this is where to begin and where to spend most of your time and energy.
Make sure the fossil is washed and clean. Put a splash of water on the coarsest 80 Grit wet-and-dry paper. You don’t need much. Now wet your fossil and put it flat slide down on the paper. Place your fingertips on the fossil and apply gentle pressure. Make sure your fingers are on the fossil and not touching the paper. It’s very easy to accidently grind away your fingertips.
Now move the fossil around in a "figure of eight" pattern. Don’t press too hard or you risk breaking the fossil if it is thin or tearing the paper. The coarsest grit (80 Grit) is the most important for making sure the surface is flat and to remove any marks left by the saw. Our advice is generally to do as long as you think, and then the same again! As you work the water will go muddy. This is the rock grinding away to make fine mud and is a sign the rock is getting smoother. The same process produces sand on a beach.
A splash of water applied to the wet-and-dry paper. For this ammonite, we are starting with the 220 Grit paper as it is already a very smooth surface.
Holding the fossil taking care to keep fingers away from the wet-and-dry paper. The figure-of-eight motion is to make sure that the wear is even across the surface.
Don’t try to rush any of these steps. If you have a deep groove, you need to remove it using the coarse paper. The fine papers grind away too little rock to remove deep scratches because the grit is finer. If you move to finer paper too soon, the scratches will remain. If that happens - don’t worry! You’ve lost a little time but gained some experience, just go back to a coarser paper (if the scratch is small you won’t need to go right back to the start).
2. Finer and finer Wet-and-Dry Paper
When you think it should be ready remove it from the paper and give it a good clean in fresh water. Be especially careful with sectioned ammonites with hollow calcite chambers. Coarser grit can get trapped in these areas. A soft toothbrush is a good tool to clean these areas but remember fossils can be brittle. It is important to remove loose grains as they will leave scratches if you transfer them to the finer paper. Now run your finger over the flat surface. If you can feel any obvious ridges or grooves you need to continue on the coarse paper.
Once clean give it a gentle dry on kitchen roll or a towel and then let it dry properly. Most rocks become lighter coloured if they are dry. You want it dry to see any scratches. The coarse paper will leave visible tiny scratches, but there should be no obvious larger ones. This is more important in the later stages when you are using the finer papers. Scratches appear as thin pale lines. If your fossil seems to be done then move to the next paper. Hold the fossil up to the light and tilt it to reveal any surface texture.
Continue repeating the steps as you work through the series of papers. In succession, use the 220 grit, 400 grit, 800 grit and finally the 1200 grit paper. Take care to clean up between grit sizes, and wash the fossil in water frequently (changing as often as you need to). Use a spray bottle filled with clean water to get slurry out of cracks, hollows and hard to rinse areas.
Holding the dry fossil up to the light to inspect for surface texture and scratches. There is always more texture on the mud filled chambers of an ammonite than on the calcite filled chambers, and it's much harder to get a good polish on these.
Sometimes when the fossil is cut, it's not cut in a completely flat plane (fossils are usually hand cut). This means that the edges might have a bit of a bevel. Anything not in contact with the wet-and-dry paper will not be sanded down, and so you might need to tilt the fossil a little to work on these areas, being very careful not to overexaggerate the bevel. You can use this technique also to round and sharp edges of a stone. Care must be taken not to gouge and tear the paper.
3. Final polish with Tin Oxide Abrasive Compound
Now this is where the final touch of magic really happens. Tin oxide is a mild abrasive, perfectly suited to most fossil specimens. Use only a small amount and mix into a very thick paste. Rub it onto the surface of the fossil or stone. Keep it damp when polishing, but not so wet that it all just flies away. Spatter is inevitable so make sure you're working somewhere it doesn't matter. Safety goggles are a good idea at this stage too!
Tin oxide is best used with a bit of mechanical intervention to speed the process up! Lapidary workshops will use flat laps for larger specimens, but also rotary Dremel-type tools fitted with felt wheels. Many people have a rotary tool at home which can be fitted with a felt wheel and used for this purpose. You can also do it by hand with a felt or leather buff but it would be a lot of effort and elbow grease to get a high shine.
The polish the tin oxide achieves relies partly on the heat generated by the friction of rubbing the fossil on the felt, and so whilst it can be done by hand on a felt pad, it is much easier and faster with a rotary tool! Be patient, it takes a little while to get a great finish but it’s so worth it.
A great tip before starting with the tin oxide is to fill any ammonite chambers or holes with a soft soap. This stops them filling with tin oxide and more importantly stops fragments of loose crystal or grit creating scratches. When you finally clean the fossil, the soap just dissolves away. When you have finished give the fossil a good clean to remove the tin oxide, this can be tricky as it is so fine. A garden mister or empty spray bottle can be used to direct the water into any hollows to clean them out. Make sure you wash your hands thoroughly too before you eat or touch your face.
The end result!
Wear finger protection. You'd think you would but you often don't feel when you've rubbed the tips of your fingers off until it's too late! You also don't need to be tightly gripping the sides. The water acts as lubrication meaning any pressure can usually be applied from the top for a flatter fossil, or elsewhere for a rounded fossil.
Don't shove gritty water down the sink. Either dispose of outside or strain through a coffee filter.
If you have a lot of fossils to work on and want to do them together, go through them grit by grit rather than fossil by fossil. It'll be much quicker to do all of the 80 grit work, then clean up, then all of the 220 work and so on.
Depending on the size of your fossil, you may wish to cut up the wet and dry paper and keep some of it back for another time. The grit size of each piece of wet and dry is printed on the back, and so if a section that you’ve cut off doesn’t have it printed, write it on with a permanent marker.
Good quality wet and dry paper can be reused until there is very little abrasive grit left. You can rinse it out because it has a waterproof backing, and put it out to dry for next time. When it's dry, you might need to put something heavy on it to flatten it out.