MATERIAL AND METHODS

Ornithopod dinosaur specimens to which the cleaning methods described herein have been applied include teeth and jaw elements from the collections of the Natural History Museum, London, the American Museum of Natural History, New York (AMNH), the Peabody Museum of Natural History, Yale University, the Smithsonian Institute National Museum of Natural History, Washington DC (SM), the Carnegie Museum of Natural History, the Iziko South African Museum (SAM), the Dinosaur Isle Museum, Isle of Wight and the Oxford University Museum of Natural History, Oxford. All were cleaned using solvent gels and then moulded with a vinyl polysiloxane impression medium. Epoxy resin casts were taken from the vinyl polysiloxane moulds, sputter coated with gold and imaged in a Hitachi S-3600N Scanning Electron Microscope (SEM).

Shellac, glyptal and shellac/animal resin combinations were removed by the application of solvent gels from the occlusal surfaces of hundreds of teeth from 143 specimens, which consisted of individual teeth, teeth within jaw fragments and teeth within complete jaw elements.

Fossil teeth that were cleaned by the traditional brushing on of ethanol method were also moulded, cast and imaged.

Creating the Solvent Gel

Components:

• 200 ml ethanol (IMS)
• 200 ml acetone
• 50 ml xylene
• 20 ml Ethomeen C/25 (a polyoxyethylene cocoamine detergent; Akzo / Linden Chemicals)
• 6g Carbopol EZ2 (a water soluble acrylic acid polymer; Noveon / Linden Chemicals)
• 50 ml pure water (distilled or deionised)

Care should be taken to follow the manufacturer's instructions regarding safe use and storage of all of these products. Material Safety Data Sheets (MSDS) are available via the manufacturer's and distributor's web sites. Both Ethomeen and Carbopol can be obtained from Linden Chemicals (www.lindenchemicals.com), Ethomeen is a product of Akzo (www.akzonobel.com), and Carbopol is a product of Noveon (www.lubrizol.com). Ethanol, iso-propanol, acetone and xylene can be obtained from standard suppliers of laboratory chemicals.

The solvent gel should be prepared and stored in polyethylene or polypropylene bottles to prevent reaction between container and gel. Bottles with transparent sides allow progress to be monitored during preparation of the gel. Bottles with wide openings and tightly sealing lids are preferable.

The quantities listed above will produce approximately 500 ml of solvent gel; for smaller volumes reduce the component quantities on a pro-rata basis. The solvent gel can be created quickly for immediate use via a one stage method, but a two stage method (see below) produces a more consistent gel, allows the pH of the gel to be controlled and gives flexibility in the combination of solvents used. Standard laboratory procedures should be followed with reference to all relevant health and safety legislation. We recommend that the addition of ethanol, acetone and/or xylene to the solvent gel should be performed in a fume cupboard. However, once the solvents have been added to the gel, a fume cupboard is no longer required. The solvent gel can be used with standard air extraction systems or in a well ventilated area.

For the one stage method of gel preparation, sprinkle the Carbopol EZ2 powder onto the Ethomeen C/25 whilst stirring continuously until a uniform paste is produced. Stir in the required combination of solvents ethanol/acetone/xylene, and then add the pure water gradually whilst stirring continuously. Apply a tight fitting lid to the bottle, and shake the bottle vigorously.

For the two stage method, first prepare a Carbopol gel as follows: Sprinkle the Carbopol EZ2 powder onto pure water whilst stirring continuously, until a smooth, stiff 'wallpaper paste'-like mixture forms. This Carbopol gel can be used within a few minutes if necessary (as soon as it settles and takes on a uniform consistency) but will benefit from being left to stand overnight in a sealed bottle to allow the Carbopol to fully disperse. Next, pour the Ethomeen EZ2 into the Carbopol gel and stir until a smooth, colourless and transparent Carbopol/Ethomeen gel forms. The bottle lid can be screwed tight and the bottle shaken vigorously to aid mixing at this stage. The introduction of Ethomeen should neutralize the acidity of the Carbopol. Testing with pH paper strips should show a pH between 7 and 8. If these numbers do not result, adding more Ethomeen will increase the pH and adding more Carbopol gel will reduce the pH.

Next, mix the required combination of solvents ethanol/acetone/xylene in a second bottle, and then cut in the Carbopol/Ethomeen gel gradually. If the gel becomes cloudy or if a sticky white residue begins to form, water must be added to the gel to allow the solvents to be fully absorbed. Adding the above solvents should result in Solvent gel with a pH of around 8.5.

The advantage of using the two stage method of production is that stock Carbopol/Ethomeen gel can be made up as a first stage. Later, small samples of this gel can be cut into various combinations of solvents for testing on unknown consolidants.

Variations in Composition

Various formulations of the gel can be made by substituting one solvent for another. The above formula, which adds a small quantity of xylene to a 50:50 solution of acetone and ethanol, acts to break the cross-links in aged shellac and enables it to be dissolved. It is effective on glyptal as it takes the guess work out of which solvent was used in the formulation of the glyptal, and it will also work on young shellac (although not as effectively as ethanol alone) and so can be used as a universal formula. An alternative formulation substituting additional ethanol for the acetone and xylene (i.e., 450 ml of ethanol) is more effective on shellac that has not developed cross-links. Typically this is shellac that is less than two years old. However, depending upon the additives used in the shellac, it may still dissolve readily in ethanol after many years. It is worth testing a small area to assess how first to treat shellac.

When solvents are added to the Carbopol/Ethomeen gel, the pH can be tested. If it is too acidic, Ethomeen can be added to reduce the acidity. Whilst slight acidity of the solvent gel is not critical for use on the highly resistant enamel and dentine surfaces of teeth, this ability to control pH has great importance for the wider application of solvent gels to less resistant surfaces such as bone.

It is worth noting that linseed oil may be a component in shellac. Whilst we did not encounter a consolidant of this composition, in such instances the acetone/ethanol combination would not be as effective. Tests carried out on oleo-resinous varnish by Burnstock and White (1990) had greater success with iso-propanol.

Application of the Solvent Gel

Loose dirt should be brushed from the surface being cleaned and a thick layer (>8 mm) of solvent gel applied. A layer of stretch plastic wrap should be wrapped over the solvent gel and the teeth/jaw to hold the gel in place. This has a number of positive effects. The plastic wrap provides gentle pressure, keeping the gel in contact with the consolidant surface, it prevents the gel from spreading to adjacent surfaces, and it reduces the amount of solvent evaporation from the gel. We found that a 17 µm thick, commercial, food quality, polyethylene stretch film plastic wrap, with cling/tack on one side, worked well for our purposes, but thicker versions of the domestic 'cling film' (UK) or Saran^TM Wrap (US) should be equally suitable.

Over a period of 15 to 20 minutes the consolidant will soften, dissolve and be drawn into the gel. Typically, the dissolved consolidant (particularly shellac) discolours the gel, and, as both plastic wrap and gel are transparent, the process can be monitored. One application of gel is usually enough. If the consolidant has been applied in multiple layers it may be necessary to repeat the process. The plastic wrap can be used to scoop the gel from the tooth surface, and both plastic wrap and gel can be discarded (subject to the relevant local regulations on the disposal of hazardous waste). With shellac, a very thin blistered film typically remains that should be 'teased' away from the tooth surface. A wooden spatula, wooden dental stick or latex block (to avoid introducing scratches) will work well for this purpose. Any remaining solvent gel can be brushed from the tooth surface with a little ethanol or water and a fine soft brush.

The Carbopol EZ2 gel allows a solvent or combination of solvents to be held in suspension, preventing immediate evaporation and allowing their action to be concentrated at a specific point and over a significant period of time. Being an amine with detergent properties Ethomeen C/25 serves two purposes when mixed with a Carbopol EZ2 gel. The slight alkalinity of the amine neutralizes the acidity of the gel and the detergent properties enable the gel to sequester material dissolved by the suspended solvents. This latter quality is of key importance in that the dissolved consolidant is taken away from the tooth surface and locked into the gel. As a layer of shellac dissolves into the gel it thins and eventually reaches the point where it will distort and blister, lifting away from the tooth surface cleanly. The clean separation virtually eliminates instances of remaining consolidant and the consequent need to re-clean and re-mould/cast.

Solvent gels need to be stored, handled and disposed of as hazardous chemicals, but no more so than their solvent components. The lower rate of evaporation from gels makes them considerably more pleasant to work with. Carbopol/Ethomeen gel is not hazardous until solvents are added, so it can be made up in bulk and stored for extended periods of time. Small volumes for immediate use can be measured out as needed and the solvents added moments before use. Alternatively, solvent gel can be made up from scratch within an hour and potentially within a few minutes.