We (re)prepared a fossil crinoid that we have had in our collection for many years now. This specimen, an Onychocrinus exculptus from the Lower Mississipian (Carboniferous) of the Edwardsville Formation of Indiana, USA, was found and prepared over 100 years ago. It was most likely part of a Victorian collection.

Crinoids are found through a great deal of the fossil record, in varying states of preservation. They are also known as sea lillies, which is a little misleading seeing as they are animals rather than plants. The modern contempories of this one are alive today.

Crinoids with their stems ‘rooted’ into the sea bed. These stems are made up of little stacked discs which often separate before fossilisation.

The Crinoid Fossil and its Matrix

The specimen lies in a soft, dark-grey shale. Upon preparation, it was observed that the specimen had areas covered by bryozoans in a sort of mat. This would indicate that burial wasn’t rapid. The mass of disarticulated arm, pinnule and calyx plates bear further testament to the animal lying on the seabed for a little time before burial. It is rather interesting that the calyx and the base of the arms are still articulated, although slightly sagged, while the more fragile pinnules are more of a jumbled mess. 

While the best way of approaching such a specimen would have been to prepare it from the underside and therefore expose arms and articulated sections, this was not the decision made by those pesky Victorians. The amount of matrix is far too small to allow adequate support for such an approach. Perhaps in the future, we will set the fossil in resin and prepare it from the other side.

Preparation using Air Abrasives

We used our Vaniman Master Mobile Problast with 53 micron Aluminium Oxide powder set at a pressure of 1-2 Bars (14—30 PSI) which is lower than the recommended pressure, but worked an absolute charm. The matrix was very soft, and was only a light coating - a situation in which air abrasive preparation excels itself and pen work is often less than stellar). The hardness of the fossil was much greater than that of the rock, making it even more of an ideal candidate for this approach.

Aluminium oxide is quite an aggressive air abrasive powder to use as it is so hard (9 on the Moh’s Hardness scale - only a little softer than diamond!) and the grains are angular which therefore means that it really ‘cuts’ through the rock. We chose to use it as it flows very evenly and can be used for reasonably delicate applications at lower pressures, particularly with soft matrix and a hard fossil. The trick is to use it carefully!

After preparation, the specimen was thoroughly washed to removed all traces of aluminium oxide powder and then thoroughly dried for a few hours by a radiator. It was then coated with 10% wt/vol Paraloid B72 dissolved in acetone to not only consolidate the matrix, but also give the fossil a light varnish to heighten the contrast. It is important to avoid applying paraloid to a specimen before it has totally dried, otherwise you might be left with a milky white film on your fossil.

The air abrasion itself took about an hour, with a couple of hours inbetween rinses to allow for the specimen to dry. All in all, a fairly quick and definitely satisfactory result.

We hope you enjoyed the video and the results!