Thursday, 27 October 2011

Humerus Preparation Finished

Sticking with all things Oxford Clay and I’m pleased to reveal the fully prepared humerus of our mystery plesiosaur – and what a fine example it is. Although the bone was in pretty good condition when it was recovered, it was in two pieces and slightly scraped at both the dorsoproximal and distal ends. This was a clean fresh break that was almost certainly caused by an excavator as it passed overhead and this was almost certainly the same machine that uncovered the specimen in the first place – a double edged sword so to speak.

To be able to look at the finer detail of the bone it was obviously necessary to remove the matrix that was adhering to the specimen. It was also a requirement that the two pieces be stuck together and, since the specimen was densely mineralised and heavy, the join would have to be of considerable strength.

Because of the nature of the matrix and its “stickability”, a variety of different tools were used on the preparation and the form of the matrix dictated what tool was to be used. The amount of matrix removed mechanically was very small and primarily revolved around the proximal end and distal flange whilst the bulk matrix on the shaft and flange were removed with various hand held tools. This was necessary because some sediment tends to blend in with the bone and the external patina is easily damaged if you are not extremely careful.

One problem that I’ve not encountered with material from the clay is knowing where matrix ends and bone begins - unlike some other material I’ve been preparing. It is easy to differentiate unless you make the mistake of going too deep in the first place – then you can have a problem but, if you are careful, then mistakes should be rare. Some Triassic skull material I’ve been working with, on the other hand, is almost identical to the surrounding matrix and is slow going.

Although the bone was tough, it still needed support throughout the preparation process and I used small sand bags in combination with mutil-layered bubble wrap to provide it. The humerus has needed very little in the way of consolidation although it has been given a gentle protective coat of Paraloid B-72 and this has also been used to glue the two parts of the bone back together at a 50/50 solvent to consolidant ratio. This has made for an extremely tough bond and appears very robust.

The shaft of the humerus was stood in a container of sand which effectively held it in place and upright and, once the glue had been applied, the flange was very quickly joined on and held in place by various bits of ethafoam, which not only held the bone in place but also allowed for adjustment. The join was so perfect that I used the weight of the flange to create the pressure required and then left it for 48 hours to really harden.

The join appears to be excellent and is likely to be strong enough but if it is found to be wanting then I can always redo it and will probably add glass beads which would form an even stronger bond but I really don’t think it will be necessary in this case.

Next up is the ulna (below) and then the radius, which shouldn’t take too long to prepare and then we will be into the proximal carpals – the ulnare, intermedium? and radiale and, after that, the distal carpals.


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