Formic (HCOOH) and hydrochloric acid (HCl) were tried at different stages of preparation of the ML433 block and were not successful for gross initial preparation. During gross preparation, the rate of dissolution using formic acid was far lower when compared to the effectiveness of mechanical preparation. Furthermore, it endangered the bone most of the times. However, during final detailed preparation, such as removing sediment traces from the bone surface, acid preparation proved useful. Formic and hydrochloric acids (5-10%) were used.

Bath immersions of specimens for a period of 2 to 5 of hours in acid proved to be very effective. In such cases, tri-calcium phosphate was used as the buffer solution (see Braillon 1973, about the use of acids in fossil preparation). One should test the efficiency of the bath immersion first only for a few minutes, in order to use adequate proportions. Acid baths allow preparation in areas that are not reachable with physical methods.

 INSTRUMENT MAINTENANCE

Regular maintenance of the instruments is highly advisable: this increases average longevity, effectiveness of the equipment, and it is more secure for the user.

In order to keep the electric grinder working properly, important routine tasks need to be carried out: 1) the interior of the engine should be sprayed daily through the ventilation slots with compressed air; and 2) every six months the rubber springs in the body of the grinder should be replaced, reducing the risk of engine damage.

The separate pieces of the airscribe were oiled and cleaned every day. A jet of compressed air is particularly effective to clean each separate piece.  The stylus must always be sharp. A non-controlled experiment to test the effectiveness of a sharp airscribe stylus versus a blunt one was performed: approximately three to four times more matrix can be removed while maintaining the stylus sharp. A bench grinder is recommended to perform this task. Constant cooling of the stylus while sharpening is crucial to extend its longevity and maintain the proprieties of the tungsten stylus. To cool down the stylus, it is preferable to dip it in any kind of oil rather than water, since oils are better heat conductors, but also because carbon can be incorporated. This procedure is recommended since hot temperatures reach while sharpening the stylus can alter significantly the proprieties of the tungsten.

MOULDING AND CASTING

 Moulding

The partial preparation of the main block revealed an exquisite set of bones and it was decided to mould the block at this stage. Moulding a partially prepared block not only preserves precious taphonomical data, but also the cast is an informative way to show the different states of preparation for display purposes (fig. 9). In order to reduce costs a compromise between moulding a partially prepared block and the bones one by one had to be considered.

Fig. 9 – Main block cast of Miragaia longicollum, after being painted and mounted. The cast was painted with regular water-based colours. Note that the different pieces of the mould are not detectable, they were connected with screws, pieces of plywood and fibre glass mats with polyester resin.

Regular silicone rubber (RTV – room-temperature vulcanizing – which means it dries at ambient temperature) was used as a mould both for the individual bones and for the block. Polyurethane resin was used to cast the block (fig. 10). For individual bone casts both polyurethane resin (for small bones) and polyester resin (for larger bones) were used. Polyurethane resin is preferable for small casts because it is easier to handle: not only it hardens faster, but also when mixed with plaster it foams becoming adherent to the surface of the mould keeping the details and texture.

A general moulding procedure using silicone rubber is described in Rigby and Clark (1965) and Goodwin and Chaney (1994). However, these techniques require a large quantity of silicone, meaning more costs and risk for the bone, especially when moulding intricate-shaped bones. The technique described in this paper makes use of a relatively thin layer when compared with the techniques given in the literature, and it is therefore more economic. When compared with other materials (e.g. latex) silicone rubber, although more expensive, is easier to detach from the bone or cast and shrinks in a lesser degree (Goodwin and Chaney 1994, p. 239). It is also flexible, long-lasting and strong.    

ML433 block mould had to be made in three different parts, due to its large dimensions. A large mould is not only harder to handle, but also occupies lots of storage room. The mould and supporting moulds should be attachable to each other. To achieve this, drill through the bordering walls of the contacting areas of the supporting mould and use bolts and screws. Each piece was moulded separately by raising putty-like modeling clay walls, which separated each part of the block to be moulded. A similar procedure as explained above was performed accordingly (fig.10).

Fig. 10 – Moulding the main block: A – raising modeling clay retaining walls; B – aspect of the block after applying the first layer of silicone; C – aspect of the mould once the second and third layers of silicone were applied; D – polyester resin supporting mould.

Fig. 11 – General procedure for moulding: A – defining thoughtfully where the mould should be divided, clay is used to separate the two parts of the mould; B – application of the first thin layer; C – in the second layer thixotropic additive is added, giving a yoghurt viscosity to the silicone; D – ladies stockings can be applied directly on the mould or pieces of cotton cloths can be embedded with silicone with a spatula before proceeding to its application on the previous layer of the mould; E – aspects of the mould with all the silicone layers applied; F –plastic film should be applied before making the supporting mould; G – aspect of the supporting mould once dried; H – on the other side, the final aspect of the silicone rubber.

As Fox (2003) noted, the first decisions (e.g. establishing the mould divisions and number of parts) and steps (e.g. filling voids, adhesives used) when moulding are fundamental to avoid damage to the bone. A general procedure to mould a medium-sized bone (e.g. vertebra, fig. 11) is as follows:

Before start moulding, all the preparation work should be completely done. Thus, the bone should be well impregnated and stabilized with appropriate binding adhesives. Gaps should be filled with polyurethane putty or epoxy glue. Holes, like neural channels in vertebrae, should be closed with regular clay or modelling clay. The first layer (the most important) is composed of: hardener (5-7,5% w/w), colouring powder and thixotropic additive (0,5 to 2% w/w; 0,5% is recommended for the first layer). The colouring powder is used to check the effectiveness of stirring: the more homogeneous the colour, the better stirred the silicone is (fig.11B). Note that a temperature of approximately 20ºC works best, however it varies between different types of silicone rubber. Use a light colour pigment for the first layer, so it can contrast with the bone and the subsequent layers. It is convenient to preheat the silicone rubber (1 litre for about 1 to 2 minutes in a microwave at maximum power; 820-900W). The silicone should be slightly warmer than room temperature. Apply the second layer on top of the first when the latter is almost dried and still sticky (use a greater percentage of the thixotropic additive, so it has a viscosity approximating yoghurt; maximum: 2% w/w).

Impregnate small (10×10cm) pieces of nylon canvas (ladies’ stocking is a good material for that) using a spatula. Ladies’ stocking fabric can be used for intricate shapes. Alternatives are pieces of cotton cloth, or even pieces of anti-weed tissue. The silicone rubber can either be spread using a spatula or applied directly on the tissue with caution. Apply the impregnated tissue on the first two layers. Prominent areas should be reinforced with more layers of silicone or cotton cloth. Ladies’ stocking fabric is flexible, resistant, porous and durable; however, it is relatively hard to apply because it is difficult to avoiding air bubbles (fig. 11D, E). Cotton cloth is easier to handle but less flexible.

A fourth layer should be applied if the thickness appears to be too thin (this decision has to be made for each mould); the average thickness of a mould should be 5mm maximum.

Supporting mould casting in polyester resin

Once the silicone mould is made, and prior to its removal, a hard shell cover should be made in order to give support and keep the general shape. The technique corresponds to the following protocol:

- Cover the mould with a plastic film;

- To make the mould support, fibre glass mats impregnated with plain polyester resin are used. It is also possible to use plaster-of-Paris: a more environment-friendly solution but much less resistant and durable when compared to a polyester resin mould support;

- After stirring the polyester resin and the corresponding accelerator, the fibre glass mats should be embedded. Use an inexpensive brush and impregnate fibre glass mat pieces sweeping it on both sides. Apply the fibre glass mats onto the plastic film covering the mould; two to three layers should be enough (max 10mm), if 300g/m² mat is used.

It should be noted that one disadvantage of polyester resin is that it shrinks about 1% v/v. To solve that problem fibreglass mats impregnated with epoxy resin can be used instead.

Storing moulds

Since this technique makes use of a very thin layer of silicone rubber, some practical tricks should be carried out to prevent the deformation of the mould, and therefore the deformation of the next cast. For large bones (e.g. a bone from the appendicular skeleton) the borders of the mould should be attached with screws to a rigid polyester support (fig. 12). Fill the interior of the mould with toilet paper – for example – in order to keep the mould fitted as much as possible to the polyester support mould. Store the mould in a safe, constant-temperature room.

Effective storing moulds described in Jabo et al. (2006) were made and applied in the Smithsonian Vertebrate Paleontological collection.

Fig. 12 – Storing moulds: screws attach the silicone mould to the rigid polyester resin supporting mould.

Casting in polyester resin

The technique consists of applying three layers that, together make optimal use of the strength and flexibility of the polyester resin (fig. 13). Furthermore, polyester resin offers special characteristics which makes it useful for casting, not only it keeps all the detail from the mould, but it is also a very versatile material since it is chemically compatible with fibre glass, aluminium powder, calcium carbonate, glass microspheres, etcetera. When combined in different proportions with these additives, polyester resin can be adapted to many different circumstances.

For a polyester resin cast, the following procedure is generally applied:

- The first layer should be more liquid than the subsequent ones, giving the base coat the shiny characteristics of fossil bone, but most importantly the surface details. Furthermore, the right colour pigments mixture will have great relevance in the final appearance and visual texture of the cast. To prepare it, one should use: glass microspheres (e.g. Dacron®) working as a thixotropic powder (25-100% v/v), plus the peroxide catalyser (1-4%w/w; lower concentrations when room temperature is higher than 20ºC, and higher concentrations when applying thick layers, or in cold temperatures). In some instances other additives can be used such as calcium carbonate powder, aluminium hydroxide  powder, or glass spheres that in different quantities produce different characteristics;

Fig. 13 – Casting: A – using a pouch as a pastry bag containing polyester resin; notice the piece of steel wire inside the mould, providing a rigid structure to the cast. B – applying polyester resin; this layer serves as the base colour for the subsequent painting (photos by Pedro Viegas).

- The main purpose of the second layer should be to even out the surface. This layer makes use of the same components, but it should be a thicker, more viscous liquid. Thixotropic powder stone filler (e.g. calcium carbonate powder or chalk) has been used; 10-50% w/w;

- Spread a layer of fibre glass chopped strands on top of the second layer while still moist to structurally reinforce the first layers;

- The fourth layer is made by plain polyester resin and fibre glass mats. The fibre glass mats are impregnated from both sides on the working table (not on the cast) and then applied directly onto the other three layers. Tear pieces of fibre glass by hand instead of cutting with scissors, so the fibres are loosened. Once the polyester resin is applied, leave the mat and wait for 1 to 2 min until it becomes flexible. The mats should be applied at least 2cm over the edge of the cast. Two to three layers of fibre glass mats are normally enough (with 300g/m² fibre glass mat);

-  While the impregnated fibre glass mats are still hardening it is possible to cut them with a Stanley-knife along the edge (1 to 2h after the last layer);

- Wait a few hours to remove the cast from the mould; it still should be a little flexible. Waiting overnight is not advisable since the cast would get too rigid;

- In order to join the two parts of the cast together, one can screw pieces of plywood (5×5cm) on the inner surface of the cast with embedded fibre glass mats (5 or 10 wooden pieces per meter). Additionally, if it is too hard to hold both halves, one can use thin metal strips with holes that are screwed through the bone holding the two halves. This way, the two halves of the cast will be firmly held.

Filling with polyurethane

- Roll a piece of moist grey clay into a cylinder shape. Apply it on the space between the two halves of the cast, in order to avoid the polyurethane foaming out. The polyurethane will be applied in the next step, and if it comes out it is likely to ruin the detailed surface of the cast.

- Drill a 2cm wide hole through the cast.

- Mix a small amount of polyurethane foam and pour it through the hole. This step should be done at least three times, since an excess of polyurethane can damage the detailed surface of the cast. Wait 10 minutes and remove the clay once the polyurethane foam has reacted entirely.

Finishing the cast

- To fill the space that separates both halves of the cast, tinted polyester putty has been used successfully. It is also possible to use car body filler but it is difficult to tint the right colour and it is necessary to use much more colouring pigment powder. Preparing polyester putty requires: plain polyester resin, thixotropic powder (1000g – 25-50% v/v), gel coat (1000g 10-50% v/v), colour (pre-mixed with polyester or powder) , cobalt (2-10% w/w; it works as a catalyser), and peroxide hardener (2-3%). Mix the cobalt before the hardener;

-  Once the polyester putty is made, use a plastic or rubber pouch or bag as a pastry bag (fig. 11A). Cut with scissors the corner (5mm) of the pouch. Spread some putty in the pouch and squeeze it along the space separating the two halves of the mould;

-  Before it starts to harden use paint thinners (e.g. acetone) and a soaked brush on it. Manipulate the putty with the brush, creating detail on the cast surface as needed. Always keep the brush well saturated;

-  After it sets, remove excess polyester putty with sandpaper. A hot air gun can be used so that the putty becomes viscous again, making it possible to scrape it off;

-  To give different tonalities to the cast it can be painted in layers with a water-based acrylic. The first layer takes away the shiny appearance of the polyester resin; the second layer can be applied with a “dry brush” method. The “dry brush” method consists of: 1) spreading some water-based acrylic on a board (whitish/yellowish ochre), 2) stirring the brush on the board so the brush is coated with paint evenly, and the paint is almost dry, and 3) gently painting the cast, highlighting the natural structure of the bone.  Applying two or three more layers should result in a great resemblance to the original bone.

-  If the cast is going to be exposed outdoors, the polyurethane will require a UV- protection coat. Since the UV-protection is normally very shiny, add talc powder (5-10% w/w) to its original composition. This procedure allows complete mounting of skeletons in a quick and inexpensive way (fig. 14).

A few notes will be considered attending polyurethane resin. Polyurethane resin was firstly described for paleontological purposes by Jansen (1961). It is preferable to use polyurethane resin to cast bones no larger than 20cm. This happens because polyurethane resin is easier to handle and mix. If it is desired, anyway, to use polyester resin for small bones it is necessary to use special fillers and to control the heat produced during polymerization. Polyurethane resin, in the other hand, is much harder to handle for large surface bones due to its rapid set time, leaving no possibility to spread it evenly on the bone. If used for small bones with a mixture of plaster it has excellent results, giving the internal appearance of bone. The first two layers determine the quality of the cast but also the final appearance of the polyurethane resin. The colour of the first layer should be chosen carefully, reflecting the outer tonalities of the bone; the second layer is the base colour. When the second layer is applied – while the first layer is still wet – it gives some heterogeneity in the final aspect of the cast. This situation is desired since fossil bone does not have a homogeneous colour.

As noted, polyurethane resin has also been successfully applied but only for small casts or intricate structures. It is a two component compound, and when mixed with dried regular plaster in equal proportions it foams. If 1 or 2 drops of water are added it will have a similar effect. The stirring should be as homogeneous as possible, and for that purpose we recommend a drill with a mixer attached. Polyurethane resin is particularly effective when poured in single orifice moulds.

Fig. 14 – Mounted skeleton of the stegosaurian dinosaur ML433.

Further notes on the casting procedure

To accomplish the casting procedure successfully:

- Two people are preferred for this task; casting and moulding is a complex and difficult task, thus the cooperation between two people reveals much more effective. The division of the sub-tasks (e.g. stirring up components, brushing, applying chop strands, etcetera) should be consistent during the entire process;

- Use gas mask, latex gloves and a lab coat (see “Health and protection issues” section). Either polyester and polyurethane resin are extremely noxious while polymerizing;

- Use an air filter (e.g. Plymovent®) or dust extractors in order to keep the air safe to work;

- If it is necessary to save the brushes they should be immersed in thinner and cleaned with a spatula or metal brush long before the polyester or polyurethane resin dries up (polyurethane resin has a faster period of drying). In order to do that use a clean and plain surface, put the brush obliquely to the surface and scrap vigorously several times with the spatula until all the liquid polyester is removed. Then dip the brush in thinner for 12h. To reutilize brushes used in moulding a brush with metallic bristles, and scrap vigorously the already hardened silicone off against a clean and plain surface;

-  All of these components are easily accessible. They are widely used for fibre glass boats industries, for example.

Fig. 15 – ML433 digital cervical vertebra, all the anatomical structures can be easily understood using 3D scanning.