Mechanical Preparation

How will the specimen be used–for research or exhibit? What area will you be working on first? What section will you move onto after this? Will the specimen be left in situ in the matrix or will it be completely removed from the jacket? 

Fossils can be shattered or distorted while lying in the ground for millions of years. Do not assume that you know where the bone is–it will surprise you every time, creating what Harvard preparator Bill Amaral refers to as the "tool marks of discovery". 

It is best to keep the tool on the matrix and not the bone. Bone is almost always softer than the surrounding matrix and the tool should be kept away from the bone to avoid damage.

Scribes, chisels and other impact tools can cause damage through vibration. Pay attention to cracks in the matrix and in the bones.

Resting the specimen on delicate areas of bone can cause damage. Specimens should always be on a soft surface and may need to be provided with additional support by using sandbags or specially made support jackets.

Do not clench or force the tool. Sharp points and tool bits will remove more matrix with better control than dull tools or tools used with more force than is necessary.

Do not completely prepare just one section at a time. As the specimen is prepared, it may become more fragile; having one section completely exposed while the remainder is protected by matrix may cause that section to suffer damage.

If the specimen is left in the matrix take care to leave pedestals of matrix underneath exposed bones. Fossil bone does not have the internal strength of fresh bone.  Prepared bones floating in air tend to break with time. They can be supported by matrix that has been strengthened with consolidant or thin fiberglass strand, to prevent movement and breakage. 

Mudstone matrices on large bones can sometimes be removed by hand without damage, but prep often begins with air scribing. Air scribes are similar to miniature jackhammers; compressed air is used to push a piston back and forth at thousands of cycles per minute. There are a wide range of sizes and powers of scribes from the pistol grip hammers made to remove large amounts of hard rock to very fine tools that remove one grain of sand at a time. The choice of scribe is dependent on the type and amount of matrix to be removed and the delicacy of the specimen.

The tool is held firmly, much like a pencil, sometimes using the finger as a guide for the point of the tool. Small strokes should be used, in a regular pattern. It is tempting to dig large gouges into the matrix with the tool, but there is a high risk of digging large gouges into the specimen by doing so.  

The tool is kept on the matrix, working back from the edge where matrix meets bone. After removing the bulk of the matrix, allow the tool approach the bone to let the remaining matrix pop off. Note that in this image the point of the tool is held almost parallel to the surface of the bone, so that the vibration is doing the work and not the tip of the tool.

Air scribes produce dust as well as flying bits of rock, so proper protection–eye protection, dust collection, dust masks–should always be used. 

Grinding tools are used on hematitic specimens, and other matrices that are resistant to scribing, as well as for sharpening needles. They differ from scribes because there is a rotary motion to the tool. Flexible shaft tools such as Dremel, Foredom flexible shaft tools, or pneumatic pencil grinders often have variable speed controls, which allow fine-tuning of the speed of work.

There are a wide variety of bits for these tools, including diamond bits, cut-off wheels, and even tiny dental bits. They are used to wear down the matrix gradually to a very thin layer, which may pop off or which can be removed with hand held points. Cut-off or diamond wheels can be used cut grids into masses of matrix that can then be removed with a chisel.

Points or needles are used for microprep and for the final stages of preparing large specimens. They are made from steel drill blanks or carbide rod. Both can be held in pin vises and shaped to fit the desired use. Steel is softer and more flexible than carbide, and can be sharpened to a very fine point, for preparing tiny areas. However, the point will also wear down fairly rapidly. By contrast, carbide is much harder than steel but more brittle–the point will tend to chip.

Points must be sharpened. Usually this is done using a bench grinder, with fine and coarse grit aluminum oxide wheels for steel and silicone carbide (“green”) stone wheels for carbide. Initially the tool goes through a “rough grind,” where the rod is held almost parallel to the stone and ground to a point. The aim is to achieve a long taper, not an abrupt shoulder. 

After a rough grind using a coarse stone, the final shaping can be done with a finer grit stone or with a diamond wheel on a flexible shaft tool. Later touch-ups can also be done with a diamond stone or the diamond wheel.

Points are often shaped to form a chisel. This has the advantage of forming a sharp edge, which can be held at a very low angle to the bone to gently scrape the matrix away. 

Also known a micro-abrasive, an air abrasive can be used for matrix removal or for a final cleaning of specimens. It uses air pressure to create a focused stream of fine particles through a small nozzle. Sodium bicarbonate powder is the most commonly used material, as it is the most gentle on the specimen. Aluminum oxide powder is often used where cutting of the matrix is desired. However, it is also more forceful, so care must be taken to avoid damaging the fossil. The differences in the media are both the hardness of the material as well is its shape. Sodium bicarbonate has needle-like particles, while aluminum oxide particles are blocky and sharp. Generally, it is best to use the nozzle at an angle to the bone, to avoid cutting into the specimen.  

Air Abrasive Particles

The effect of an abrasive material is caused by its three characteristics: shape, hardness, and particle size.

Particle shape

Individual particles that are blocky shaped have points and edges that will cut and strip away surface material on impact. Spherical shaped particles have relatively no cutting ability and are used to pound or “peen” a surface.

Hardness

The hardness of a media is measured using the Mohs scale. The harder the media, the more aggressive it will be in removing material from the work piece.

Particle size

A larger particle generates a greater impact force as it strikes. Media with larger particles removes material faster. It also produces a heavier texture on the base material.

Specimens will often need consolidation and gluing during preparation. You should be aware of the properties of the various adhesives, and have them on hand before beginning to work. It is important to be aware of that glues may need to be un-glued and consolidation may need to be loosened during prep.  

There is a lot more to mechanical preparation. Preparing to prepare, supporting specimens, how to position yourself to prevent injury are additional relevant topics covered in the resources listed below.  

Access detailed information on both micro- and macroprep tools and equipment and outfitting a fossil preparation lab to create an appropriate work environment.