The Curse of the Undercut
Technology, hate it or embrace it, is responsible for bringing art to the masses.
With pictures and words, the printing press made reproductions far more accessible. For three dimensional art forms, there was the mould.
Sculptors wanting to capitalize on a particularly popular piece or architects needing to have a pattern of plaster acanthus leaves adorning a temple, no longer had to re-carve inexact replicas. Instead, they created a mould of the original and poured in a liquid medium -- like molten bronze or gypsum plaster -- waited for it to set, pulled out the fresh cast, and repeated.
Roman lamp mold carved from limestone.
Early printing press.
Column capital with undercuts beneath leaves and volutes.
Enter the Undercut
Printing presses, as all mechanical devices do, had their breakdowns, but generally, if given enough type, ink, and operators, they could produce thousands of pages a day, even as early as the 15 century. But since its inception, the mould had a bothersome fly in its ointment, the undercut.
The technique of undercutting on a sculpted piece of art involves removing material so as to create an overhanging or a concave portion.
For the artist, it causes more play between light and shadow. For the mould maker, it causes a problem. Undercuts “lock” a cast in its mould. It can’t be released unless damage is done to the cast or the mould.
Bravo, Braveau!
To illustrate, let’s take the case of an entirely fictional sculptor, let’s call him, “Monsieur Braveau”. His most famous sculpture, “Küb”, was a cube. To create a mould, Braveau simply poured liquid plaster over the top, on the sides, leaving the bottom -- the side resting on his work table -- unplastered. Once it dries, he picks up the whole thing and slides the sculpture out the open bottom.
(Yes, there are more requirements for de-moulding a sculpture, such as greasing the sides of it before applying the wet plaster and making a small vent or opening in the top to avoid vacuum locking the cube in the mould, but let’s take one critical problem at a time.)
Behold the aesthetic glory of… Küb!
The mould (blue) is slid up and off the sculpture.
Block Locked
Braveau sells innumerable copies of Küb which eventually devalues it. Braveau’s partying ways force him to create his next masterpiece, a progression of the first. It’s another cube, with a much smaller cube on one the sides. For this “Küb avec parasito”, Braveau immediately goes through the same steps to create a mould.
This time, however, when he tries to slide the plaster off, it doesn’t budge. That tiny additional cube has formed an undercut that the mould can’t slide over. Hopefully, Braveau was able to chip off the plaster mould without damaging his precious piece beneath.
Küb avec parasito.
Mould (blue) is locked by the undercut (arrow).
Pieces of Relief
A low-relief frieze.
Despite this limitation, or because of it, sculptors and mould-makers discovered very inventive ways to overcome the undercut.
The most direct was to create pieces that had no undercuts. Low-relief sculptures and carvings were prime examples and already in vogue when the concept of moulds first occurred.
Other solutions included moulding the bulk of the sculpture’s shape, finished except for any undercutting. Craftspeople would then take the cast and then carve the overhangs and undercuts. Finicky parts could also be removed before moulding, moulded separately, and re-attached to the final cast.
Piece moulds are another method. Instead of a mould that was a single piece, like Braveau’s cube mould, artisans painstakingly made several smaller molds that fit together like a puzzle. When a cast was ready to be removed, each mould piece was removed individually. Braveau’s second sculpture could have easily been moulded with a 2 part piece mold.
All of these fixes helped artisans create exact or near exact replicas, and they were certainly faster than carving an entire piece over and over again, but the human species is never satisfied with “pretty good”.
An eight part piece mould.
Chemistry Has The Answer
Examining the problem, it was obvious that a single piece mould made of a rigid substance on a sculpture also made from a rigid material could not be removed, if there were any undercuts. Logically, if there was such a thing as a flexible mould, the undercut would cease to be such a nemesis.
In the mid 1800’s, Monsieur H. Vincent was credited with using a single gelatine mould to create six casts before it broke down. Plaster pattern makers often refer to gelatine as “glue”, probably because it’s made from the rendered hoofs and bones of horses and cattle. Because gelatine can be bent without snapping or tearing, it can be pulled away from undercuts.
Gelatine moulding remained the primary technique for plaster studios into the late 20th century. But it wasn’t a panacea.
To reduce it to a pourable liquid, gelatine needed to be melted in a huge boiler. After it set, the mould needed to be treated with wax and kerosene. To improve its durability, formaldehyde was applied. Because it was sensitive to heat and setting plaster is hot enough to cause third degree burns, a gelatine mould might reach its expiry date early, leaving mould makers to clean handfuls of goop from a casting. After a few casts were made, the degraded moulds had to be re-melted and poured again.
In short, gelatine moulds were better, but far from the best.
An example of gelatine moulding from William Millar’s “Plastering, Plain and Decorative”, 1905
Pouring a silicone mould.
Silicone Ally
Enter silicone polymers. With their properties of water repellency, virtually no shrinkage, low surface tension, low adhesiveness, and resistance to high temperatures, moulds made from silicone rubber may be the best plaster studios can ever hope for. And because they can last in excess of twenty years, studios can have a library of moulds, ready to be poured at a moment's notice.
Releasing a cast from a silicone mould.
Digitally and Beyond
To be certain, chemical tech has proved a boon to sculptors and mould makers. Now, digital technologies may threaten the hand-crafted expertise that has been passed down for millennia.
Equipped with a laptop and a 3D printer, today’s M. Braveau would never know the perilous profile of an undercut. He could offer his Küb as a 3D model online and reach every art buyer in the world. All without ever once having touched his own creation.
3D printed silicone moulds.