-Polymer acrylic resins of
the brands Paraloid (Acryloid) B72, Plexigum PM381 or equivalent,
dissolvable in acetone. Plexigum is a 2-Methylpropyl 2-methyl-2-propenoate
-Epoxy glues: Ceys® or
Araldite®, two-component epoxy;
-Paleobond® PB002 penetrant/stabilizer;
obtainable at http://www.paleobond.com/;
adhesives; obtainable at http://www.paleobond.com/;
- Paleobond® PB303 activator
non-aerosol; obtainable at http://www.paleobond.com/;
-Paleobond® PB400 debonder
solvent; obtainable at http://www.paleobond.com/;
-Carbowax® 4000 C005
(polyethylene glycol, storage temperature: 20ºC); obtainable at http://www.dow.com/polyglycols/carbowax/;
-Starbond® Thin EM-02;
obtainable at http://www.starbond.com/;
obtainable at http://www.starbond.com/;
-“Polyester putty (11-18% w/w styren; takes
about 20min to dry; hardener paste: debyzoylperoxide or dibenzoil 50% w/w;
aluminium powder/silica spheres.
carbonate powder can be added in different proportions); obtainable at
- Formic acid (HCOOH);
obtainable at chemical stores.
-Rhodorsil® RTV 863 N
silicone; obtainable at http://secure.silmid.com/;
-Catalyst 863 N catalizer;
obtainable at http://secure.silmid.com/;
-Rhodorsil® ADITIVO PC 12
(0,5 to 2,0% w/w) thixotropic additive; http://secure.silmid.com/
-Colour pigment powder;
obtainable at a ceramics supplier;
-Ladies stockings (also known
as collants) or anti-weed tissue (type of fabric used to avoid growth
-Paint thinner; obtainable in
a chemical store;
-Polyurethane resin SG
130/PUR11 (stiffening time: 20-40min; recommended temperature of usage:
18-25ºC; aluminium powder and pigments can be added); obtainable in a
-Polyester resin (11-18%
styrene; takes about 20min to dry; hardener: debyzoylperoxide or
methylethylketon peroxide); obtainable at most chemical stores;
-Fibre glass chopped strands;
obtainable at most chemical stores;
obtainable in a chemical store;
-Grey clay; obtainable in an
-Polyurethane UV- protection
coat; obtainable in a chemical store;
-Polyurethane and water based
acrylic (e.g. Galeria®, Rembrandt®); obtainable in an artist store;
-Peroxide hardener; sold with
obtainable in a chemical store;
-Screws, wooden and metal
-Aluminium powder; obtainable
in a chemical store;
-Silica spheres; obtainable
in a chemical store;
-Fibre glass mats (300g/m2)
and strands; obtainable in a chemical store;
-Calcium carbonate powder;
obtainable in a chemical store;
-Colour pigment powder;
obtainable in a chemical store;
-Paint thinner; obtainable in
a chemical store;
-Plastic wrapper film;
-Brushes; obtainable at art
Security and health:
- First aid kit;
- Protections for eyes
(goggles) and ears;
- Respiratory protection
- Field boots;
- Lab coat.
FIELD EXCAVATION AND POLYURETHANE
collected from the Miragaia Unit of the Lourinhã Group (see Mateus et al.
2009, and references therein). The stegosaurian dinosaur remains were found
(fig. 4) along a secondary road linking the villages of Miragaia and Sobral.
The construction of this road was the cause of destruction of the posterior
part of the specimen.
excavation and collecting of ML433 was conducted by the Museum of Lourinhã
during two digging seasons in August 1999 and another in August 2001. During
the first season, the quarry was opened (towards the west) with the
assistance of nine volunteers. several isolated bones were extracted (fig. 5
and 7), the main block containing limb bones, ribs, pectoral girdle and
cervical vertebrae was trenched but not removed until the following year.
Fig. 5 – Field techniques: A – site where the
specimen was collected before excavation; B – first season of excavations; C
– isolating the main block; D – jacketing technique: applying the metal
frame, the block was already protected with polyester and plaster; E –
applying polyurethane on the metal frame, once it hardens it attaches to the
main block; F – aspect of the quarry once the main block was removed; G –
transporting the main block to the museum.
The bone-bed is composed of compact
calcareous sandstone overlain by more than 45cm of the sandstone with
lignite lamina and carbonate nodules. The layer is subsequently overlain by
50 to 130 cm layers of mudstone, some being fossiliferous (fig. 6). The
fossiliferous layer was hard to quarry, since the matrix was very dense and
the preserved bone was generally more fragile than the surrounding rock. In
the other hand, the cleavage between rock and fossil was perfect, enhanced
by its brittle characteristics.
Fig. 6 – Schematic stratigraphic
log of the excavated area.
second season, the block (then with approximate dimensions: 1,5m × 1,5m ×
1m) that had been isolated during the first season was recovered from the
field using a jacketing technique with expanded polyurethane (see fig.5 D,
E; 7). Initially, a layer of acrylic impregnator was applied to the bones,
followed by aluminum foil or wet paper. This impregnator layer, applied with
a small brush (0,5cm diameter), not only prevents the surface of the freshly
exposed fossil to be effaced under mechanical preparation, but also
impregnates and consolidates the interior of the fossil bone. Special care
should be take when the solvent is evaporating (normally 2-3minutes), since
it weakens the bone while it did not evaporate completely. The solvent used
is acetone. Subsequently, plaster-of-Paris with gauze was applied on the
foil, although burlap has revealed to be more resistant and cheaper. Wood
boards and a metal frame were placed on top of the block, increasing
rigidity and at the same time making the block easier to handle. After this,
regular cardboard was placed all the way around on the lateral parts of the
block, serving as retaining walls to control the liquid polyurethane. As the
liquid grows and solidifies into foam, the wood structure gets embedded
within the polyurethane and firmly attaches to the block. Once the cardboard
is removed, the mushroom-like monolith can be flipped over securely, after
the base is totally excavated. The expansion of polyurethane liquid into
foam takes a few minutes and grows to between twenty to thirty times its
original volume. Attention must be paid when using polyurethane: the air
temperature and the proportions of the two different components used will
lead to different reaction time and polyurethane foam proprieties. Hot air
temperatures induce a faster reaction and distinct proportion of the two
components will provide a more flexible, or more brittle foam. The adhesion
to the adjacent surfaces also changes with the proportions. Presence of
water will enhance the expansion effect, one or two drops each 20cl are
enough to produce this effect).
polyurethane jacketing technique is particularly effective in comparison
with plaster-of-Paris-only jackets when the blocks are heavy or have
irregular shapes. Also, whereas a rotational cutter is mostly needed in
order to open a traditional plaster-of-Paris field jacket, a polyurethane
field jacket can be opened very quickly and easily using a large knife or a
structure absorbs physical shock and gives a strong cohesion to the block.
In sum, the advantages of combining polyurethane jacketing with regular
plaster-of-Paris jacketing are: 1) lighter weight (especially relevant in
big monoliths), 2) the foam adapts perfectly to the block shape and fills
open gaps, 3) the flexible foam absorbs shocks and gives cohesion, 4) the
original liquid net volume is smaller when compared to the expanded foam,
thus easier to transport and storage and 5) polyurethane foam can be reused
as packing material, for example.
quantities of polyurethane bits that the opening of the field jacket will
produce can be recycled. They can be used either as packing material or as
filling for casts. By pouring freshly mixed polyurethane foam over and in
between the bits will lock the two halves of a cast firmly and will
dramatically decrease the amount of new polyurethane used to foam out a
disadvantages, the polyurethane is 1) more expensive, 2) less available, 3)
toxic after setting if the reaction is not complete (the dust is irritating
to the skin and eyes), 4) it is combustible in its solid form, and 5) it is
less controllable and manageable than plaster-of Paris ( the liquid is
harder to control than a putty consistency).
techniques are seldom reported; Leiggi et al. (1994), Schulp et
al. (2001), Watabe et al. (2004) are some of the rare exceptions.
Leigii et al. (1994, p. 75) described the traditional plaster
jacketing technique. Schulp et al. (2001) is reported a technique
where a steel collar was welded around a large block in order to remove it.
One of the disadvantages of such a technique is that it requires welding in
the field, which necessarily increases the logistics required. In Watabe
et al. (2004) it is described a technique that although very safe for
the block requires large quantities of plaster-of Paris.
Fig. 7 – Field map. Each square scales 1m2 in
the field. Black – isolated elements; Grey – material composing the block.
LABORATORY TECHNIQUES: MECHANICAL PREPARATION
airscribes and an electric grinder (also known as angle grinder) were used
during the preparation of ML433. This revealed to be very effective in the
initial stages of block preparation due to the extremely hard matrix
composition. The use of a high-speed rotation electric grinder instead of
the traditional hammer and chisel reduces vibration and damage to the
specimen. The conditions that allow the effective use of these tools are 1)
the contrast in colour between the matrix and fossil bones (in this case,
the bone is black and the matrix is whitish, making it easy to use such
heavy tools), and 2) the good preservation of the skeletal elements. Large
quantities of matrix can be removed quickly using the electric grinder with
minimal risk for the bone, but it is only advisable for the early stages of
dismantling the block elements.
The use of
the electric grinder releases a large quantity of rock dust, so working with
dust extractors and in highly ventilated areas is recommended. Good dust
proof masks, gloves, plastic.
techniques can be applied at different locations on the block relative to
the position of the exposed bone:
“groove and break” (fig. 8A) is used when there is a high probability that
what is being removed is exclusively composed of matrix. The grinder is used
perpendicular to the matrix and 1 to 3 cm furrows are cut, producing a
series of parallel grooves. The remaining matrix can be destroyed with the
disk parallel to the matrix or with the airscribe, depending how close the
exposed bone is. A long and pointed airscribe stylus should be used when
applying the “groove and break” technique.
“polishing”: with lateral movements on the matrix, slowly pulverizing the
rock. This technique is used when unexposed bone is expected below the
surface of the matrix (fig. 8B). It is still possible to use the electric
grinder very close to the bone, but expertise is required when handling the
grinder. This technique should only be performed by experienced users. It is
important to have a side bar on the grinder to enhance grip, maneuverability
and security. The grinder is held almost parallel to the matrix and hands
must be firmly supported against the block. As the rotating disk gets closer
to the matrix, thin slices start to split from the block, and the bone
remains preserved. If it is not possible to reach with the electric grinder
or it is too dangerous to use this tool, then the airscribe can be used as
an alternative. However, using the disk parallel to the matrix might enhance
risk, since unwanted pressure is developed. Nevertheless, most of the
grinders are designed for lateral pressures.
One of the
disadvantages of this technique is that once a mistake is made, it is
irredeemable, unlike the case when using chisel and hammer, as it is still
possible to glue the pieces back together. On the other hand, the time spent
during preparation using the electric grinder is much reduced. Because this
technique releases large quantities of dust which obscures the bone it is
important to keep the block as clean as possible (with an air exhaust
system, vacuum cleaner, or compressed air pistol).
In a second
approach, when the block was dismantled and the bones individually
separated, lighter tools were used, namely: sandblaster and micro-airscribe.
Both of these tools are widely used among fossil preparators, but during the
preparation of ML433 they were used in very specific circumstances (e.g.
when dealing with fragile bones, such as the dermal plates). Most of the
bones have a perfect cleavage between the bone and the matrix.
Fig. 8 – Mechanical preparation techniques using angle
grinder: A- “groove and break”, notice the perpendicular position of the
angle grinder relative to the matrix ; B- “polishing”, aspect of the matrix
after applying the “polishing” technique”.
LABORATORY TECHNIQUES: ADHESIVES AND CONSOLIDANTS
Consolidants have been used
during mechanical preparation, impregnating the bones which increased
cohesion and strength to the bone. And, adhesives were used most of the
times post mechanical preparation, in order to glue separate pieces and
Consolidants and adhesives were
widely used. Acrylic polymer resins diluted in acetone were mainly used were
initially (brands: Plexigum® a product of the German company Rohm, Osteofix®,
and Paraloid B-72), and subsequently the cianoacrylates. Consolidant
concentration normally used was 10%v/v. Acrylic polymer dissolved in acetone
was applied to the surface of the bones, which served as an
impregnator, and cyanoacrylates were used as adhesives. Two-component epoxy
adhesives were applied when the areas to be glued were larger than 10 cm3,
or when gaps were present between the two surfaces. Epoxy glues seem to be a
better gap filler since it has have stronger binding proprieties, although
it takes longer to dry (4-8h). However, cyanoacrilate adhesives were
preferable than epoxy glues, since it allows a more controlled handling. The
epoxy glues tend to oxidize superficially and turn into an altered yellow
colour with time (two years or more), which does not happen with acrylics.
Polyethylene glycol was used successfully, serving as a rigid reversible
(soluble on water and liquefiable by heat) base to prepare the very fragile
skull bones and osteoderms.
LABORATORY TECHNIQUES: ACID PREPARATION
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.
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.
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
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
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
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).
– 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.
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
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.
casting in polyester resin
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.
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.
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
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.
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
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
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).
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
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
accomplish the casting procedure successfully:
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;
gas mask, latex gloves and a lab coat (see “Health and protection
issues” section). Either polyester and polyurethane resin are extremely
noxious while polymerizing;
an air filter (e.g. Plymovent®) or dust extractors in order to keep the
air safe to work;
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;
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.
In order to
have anatomical information in digital format, the 14th cervical vertebra of
ML433 was digitalized using a laser non-contact digitizer scanner Minolta
Vivid VI-910 (fig. 15) with the assistance of the company Scorzio/B’Lizzard
Ltd. The scanner combines the 3D surface information, acquired by a laser
beamer, and real colour acquired using an incorporated photographic camera.
The information obtained during each scan was a polygon representing the
bone surface from that perspective. The vertebra had to be scanned in
multiple perspectives in order to gather information on all sides and
surfaces of the bone. Each individual scan was merged into a single analysis
that gathered all perspectives, providing a complete tri-dimensional digital
vertebra file can be exported to several 3D file formats including *.stl,
which allows rapid prototyping into real 3D. An optimized polygonal version
was saved into an executable file permitting easy access and visualisation.
The data in
the post-processing stage covers a large number of possible options. The
basic options are essentially aligning and merging the unprocessed data into
a complete, solid triangle mesh. The final step in the post-processing
procedure is to export the completed mesh into the 3D file format and
technology has multiple uses:1) to produce objects appropriate for
cost-effective rapid prototyping; 2) to generate *.cnc machining paths from
the object; 3) to generate 2D images of the object with optimal lighting for
use within publications or on web sites; and 4) to generate 3D objects with
optimal lighting for use within web sites, for example.
1) the scanner is portable and compact, it is possible to use it outdoors
(although uncontrolled daylight is not ideal for scanning); 2) because it is
a non-contact scanner the output is quickly acquired, being more effective
and ideal for sensitive objects; 3) the resolution of 307.000 points each
2,5 seconds allows fast digitizing in very large objects; and 4) the file
with the scanned bone morphology is easy portable.
Disadvantages: the data processing requires specialized skills that are time
consuming or costly.
laboratory work raises some health issues. Some suggestions are given here:
Keep a complete and updated first-aid kit always easy accessible.
Users of heavy equipment, such as jack hammers and angle grinders,
should have adequate training and supervision, and always wear suitable eye,
ear, hand, and respiratory protection. Adequate clothing should be used as
Chiseling should be done with large-headed hammers. Classic geological
hammers are not suitable for chiseling.
Polyurethane should be handled with hand and respiratory protection.
Acid baths should be conducted in a fume hood, with skin, eye and