Making Replica (Dino Kingdom)

REPLICA

-Making a reference specimen-

The Nakasato Dinosaur Center is also in charge of making replicas (casts) of Mongolian dinosaur fossils which had been displayed in the special exhibition of Mongolian dinosaurs. Though the real fossils will be returned to where they originally belong to, their elaborate replicas will remain in Nakasato.
Those replicas are made not only for display but also for research. They are to be used as reference specimens.
Experienced replica makers are relatively scarce in Japan, however, we hope that our activity will improve such situation.

Reference specimen for dinosaur research

Every fossil is valuable because it is the only thing in the world and sometimes cannot be replaced by any other one. But it is meaningless if fossils are just safely kept away from people. They should be investigated by scientists as much as possible.
There are some problems on fossil research such as;

original fossil cannot be investigated by many researchers at the same time.
some fossils are so fragile that there always is a fear of damaging the rare and important materials during their transfer and investigation.

If there is a replication which has the same characteristics (ie: shapes and colors) to original fossil, it can be subjected to various investigations in the same way as those using original fossils. Such replication (replica) which helps fossil research is called "reference specimen".
The reference specimen provides many advantages to fossil research like followings;

more than one scientists can access to rare specimens at the same time.
original specimens can be kept in safe place whenever the research which requires real fossils, such as fossil weight and composition analysis, is not performed.

Accurate replicas are inevitable for fossil research for these reasons.

Activity of Nakasato to train replica makers

However, history of dinosaur research in this country is relatively short (less than 30 years), and it had been long believed that Japanese geology would not produce dinosaur fossils. Original fossils had been scarcely found, and therefore the requirement for reference specimen had been almost negligible. But recent findings of many dinosaur fossils and footprints in this country are getting to draw the attention of people.
Today there are relatively few experienced replica makers in our country.Therefore foreign techniques are sometimes required to obtain accurate replicas, however, it would be often a time and money consuming process.
In Nakasato, not only the dinosaur fossils but an experienced fossil preparator was also invited from Mongolia to transfer their techniques for mounting and replicating dinosaur fossils to Japanese trainees. The actual trainees in Nakasato were painters and sculptors.
This activity might provide other scientists outside of Japan access to rare Japanese dinosaur specimens by assisting to make accurate replicas by ourselves. And we really hope this would contribute to a progress of dinosaur research.

Replica production method

A replica for research use is required to have detailed replication of the original fossil. And, not to mention, special attention should be paid to prevent unnecessary damage on an important fossil.

Materials used for molding

Gypsum is often considered as a molding material, however, silicon rubber is actually used for the purpose just because silicon provides many advantages such as;

it can accurately capture and replicate very small and fine features in the fossil
flexibility of the compound facilitates duplicating a complex-shaped specimen without unnecessary damage

As thin silicone rubber can not provide enough toughness to the mold, gypsum is used as the mother mold to support it.

Multipiece molding

In multipiece molding, number of parts of the mold should be first determined. The simplest one is a two-piece mold where upper and lower half is separately molded. As some specimen, such as hollow skull or pelvis, is morphologically very complicated, entire mold is divided into four to seven parts in those cases. If there is a deep undercut or orifice in the fossil, a small mold is specially fabricated for the particular part of the fossil. Deep experience should be needed to determine how to devide a mold into pieces.

Molding and casting method

Now we are going to look into the actual molding and casting process with an example of a two-piece molding, the most basic one.

	Constructing a clay base First construct a clay base around the bottom of the specimen. This base is later covered by silicone to make first half of the mold. Care should be taken not to leave space between clay and the specimen to avoid forming extra rubber or debris between them. Note that the seam line should be at the same level of the most laterally protruded pa rt of the specimen. Consequently, depending on the morphology of the specimen, surface level of the clay base is not always horizontal. This clay base should be carefully fabricated because it affects the easiness of separation of the mold from the specimen, otherwise the mold may be torn or the fossil may be damaged.
	Painting silicone on the specimen Three things prior to brushing silicone onto the specimen; Firstly, make small dimples (keys) around the perimeter of the specimen which enables a preparator to later put both halves of the mold together at correct position. Those dimples being spotted around the specimen in the clay base can be seen in the left figure. Secondly, molding compound (silicone) is prepared. Amount of silicone is calculated from the surface area of the fossil, and poured into a bowl. Activator, which catalyzes and controls curing of silicone, is added and thoroughly mixed. Amount of activator is 1% of silicone. Lastly, release agent, potassium soap, is applied onto the surface of the fossil which facilitate demolding. Now it is ready for brushing silicone to make the first layer of the mold.
	Applying second silicone layer The first silicone layer cures after approximately two hours, then another silicone layer is applied. Usually gauze is sandwiched into the layers to reinforce the mold. In addition, small silicone cubes are attached to the surface which anchor the gypsum mother mold. The brush used for silicone painting should be cleaned with benzine.
------	Applying gypsum to make a mother mold After the second silicone layer cured, gypsum (plaster) is applied to support the silicone layer. Gypsum is also coated twice with reinforcement (linen cloth) sandwiched. It usually takes 15 minutes for curing. Upper half of the mold is now completed.
	Making a lower half of the mold Next, another half is fabricated. The mold is turned over to remove the clay. A series of silicone coats is applied to the uncoated specimen and followed by a plaster coating as is same as the first half molding. After all the layer cured, both halves of the mold are separated at the seam line to take the fossil out from the inside. This is quite a nervous operation since fossils are usually very fragile. First the mother mold is removed, then thin silicone rubber layer (detail surface) is gently peeled off from the specimen. The silicone rubber is put back into the mother mold and entire molding process is now completed.
	Casting with polyester resin The mold is coated with polyester resin to make a replica. First of all, casting compound should be prepared. Three types of formulations (spring + autumn/summer/winter) are commercially available, with which adequate curing time can be obtained in each seasons. Resin is poured into a bowl, then activator (2.5%) is added and well mixed. Casting is performed by brushing polyester resin onto the inner side of the mold. Care should be taken to let the resin penetrate into every details. After the first layer cured, fiberglass is sandwiched as a reinforcement, then another polyester resin coat is applied. When coating both halves of the mold is done, those two are put together before coated resin fully cures. Both halves are tied with elastic bands to seal the seams completely. Though the cast is hollow, polyester resin is strong enough to support the cast. The brushes used for coating resin are thoroughly washed with acetone.
	Pulling out the cast from the mold The cast is removed from the mold after the resin cured (30-40 min.). This step also requires special attention to prevent damaging the cast. The cast is now ready for painting.
	Painting the cast Cast painting is performed with referring to the original fossil. Care should be taken not to cover fine details of the fossil with thick paints. Even the thickness of paints should be controlled to duplicate details of the original fossil if the cast is to be used as a reference specimen.