The best method for making museum-quality reproductions of popular sculptures is through mold making and casting.
The first step is to make a negative mold of the original sculpture. This is nothing but a hollow model that captures the form, lines and other details of the three dimensional sculpture. It can be made with different materials such as plaster, gypsum, alginate, moulage, clay, wax, polyurethane rubber, silicone rubber, latex rubber or even simple plaster bandages.
Various techniques are used for making a negative mold. The choice of technique will vary as much on the type of sculpture and mold making material being used as on the skill and comfort level of the mold maker.
For instance, the mold making material can be poured on the sculpture or brushed on it like paint (at times, successive coats may be required when using latex rubber). Materials like clay or wax can be gently patted on the sculpture to form a mold. The sculpture can even be dipped into a liquid mold making material to form a spitting negative form.
A lot of technicalities go into making a mold. The mold may be made as a single piece or require two or more parts depending on the shape and undercuts of the sculpture. You may need a mold box; spues and keys have to be made for two-part molds and an appropriate release agent is almost always required. Keep a watch out for air bubbles as they can mar the finish and details.
How do you know whether the mold is ready? It has to be allowed to set and cure properly before demolding from the sculpture. Specific checks let you know when the mold is done. Demolding requires particular care to avoid cracks/tears in the mold or even damaging the original sculpture in any manner.
Now it moves to the casting stage. Again, there is a choice of casting materials such as plaster, polyurethanes, latex rubber, silicone rubber and so on. You can even use cold casting powders to simulate the look and feel of real metals.
For casting, the chosen material can again be poured into or brushed on the mold. An appropriate release agent is generally required. You may even need to add fiber to the material to control the flexibility or strength of the cast or even fillers to alter the weight of the finished cast.
After proper curing, the mold/cast is cut or peeled away to reveal the final cast. However, it is not yet ready. Further finishing is essential to give it the look and feel of the original sculpture. Some artists even create faux oxidation for replicating iron sculptures.
The same procedure is also used to make copies of regular models and other items too. These reproductions make excellent gifts and are habitually used for sale as well. The materials used can be varied such as plaster, polyurethanes, metal or other casting rubbers.