Series introduction. October 2009.
By Nicholas Bogosian
Modeling, Molding & Casting
Molding compounds, long ago, were made from animal by-products. The molds would, in turn, attract all sorts of vermin. The shelf-life of the mold was brief. Today, the mold maker can still be found – in fabrication plants, in art studios, in special effects labs, and in the preservation trades, to name a few.
Since the days of edible molds, we’ve come quite far in our scientific development of more durable and lasting molding materials. Today the mold maker can select from polyurethane and silicone liquid rubbers as well as latex, alginate, and wax. The decision on which to use is not a mere preference, but rather dependent on what material you will be casting with, as well as the shape characteristics of the piece. The litany of casting materials is much more extensive: wax, concrete, plaster, epoxy, polyurethane, polyester, acrylic, and metal. Along with casting material and the shape of the desired piece, there are many other factors to consider before choosing your materials. These considerations can be found in molding and casting materials catalogues.
Model and mold making, for the preservationist, can be one of the few avenues to be creative and artistic, especially if he or she is given a restoration job. Say a Federalist style home has had many occupants through the years and many additions – and say one of those additions was dropped ceilings in a front office for a realtor. After research has been done on the property, it is decided to restore the ceiling to its characteristic decorative plaster ceilings which no longer exist, complete with an elaborate plaster medallion. It is then your job to create the Federalist ornamentation from scratch, with the aid of photographs and diagrams of the period’s style.
In a recent project at Belmont Tech, we were to find some section/piece of decorative architecture (whether in print or in real life), render the example, model the example from clay, create a mold of the model, then cast the mold with plaster.
I found my example out of a Gothic Architecture book – a small section from a c. 1500s woodcarving that encased a window. Then I rendered the photograph into an image that was the size I needed.
Second, with my design, I needed to roll out the clay to get a uniform thickness.
“Tracing” the image onto the clay is done simply by using a modeling “poker” to poke holes in the clay along the lines.
Then the process of carving out the image begins. Here, ready for touchups:
The degree of detail that one pursues on such complex modeling designs will be dependent on the time available, and the placement of the object in the structure. Our professor gave an example of someone trying to remove each and every fingerprint from the clay for an enormous medallion in a historic theatre which will be not only in half-light most of its life, but nowhere near enough its admirers for fingerprints to be seen. And at this rate, the preservation artist ends up making barely twenty-two cents an hour!
Next, a clay dam is created around the model to contain the molding compound as it cures.
Because my model had fairly deep crevices, lips, and some delicate shapes, it was best to go for a molding compound which would be soft enough to maneuver from the plaster once cast. I used a 74-30 Polyurethane Liquid Rubber, which has two parts: the 74 classifies the resin and the 30 classifies the hardness. Every molding compound has specific instructions for preparation. The two parts are designed to produce a chemical reaction when mixed, and will only do so if mixed properly. This particular polyurethane was a one to one ratio. The molding compound is poured into the dam and allowed to cure for a day.
Once the mold is retrieved, gypsum casting plaster in powdered form is mixed with warm water till a dip of the finger shows no skin. Once the mold has been sprayed with Spray-Release, the plaster is poured into the mold. The plaster should not sit in the mold for more than a day, as it will be more difficult to remove. The still-wet plaster casting can be removed after a half-hour and left to cure in the open.
Brush-on molds may also be utilized in pieces which are still attached, such as decorative cornices, capitals, or lion’s heads. In these situations, there would be no way to remove the object, and there would be no need to.
Having the technology of model and mold making makes the preservationist’s job efficient and more cost-effective because of the variety of materials at your disposal. If actual decorative pieces can be retrieved and molded, the modeling step is taken out altogether. Once a mold is created of a single object, it can be duplicated easily for repetitive patterns and used for many years to come.