Fancy that – the chemistry of costume masks

Most costume masks are made from liquid latex. Around a third of this material is rubber from tree sap, which is filled with long chains of cis-1,4-poly(isoprene). The rest of the mix is water and about 1% ammonia, which controls the pH of the solution. The long polymer strands are the ultimate stretchy material and are usually found curled up in their resting state. Pull them, however, and it’s like tugging out pasta from a bowl of spaghetti – they can stretch a long way before springing back to their original shape. This property means that, once set, a rubber mask has a lot of give.

The process of making a latex mask is relatively simple. First, you create a core – a solid piece shaped like a head – and you sculpt the face of the person or creature you’re trying to create using clay. You can also take an impression of a real person’s face, called a life cast, and use that to make a core. With a simple mask, you can paint on layers of latex when your creation is ready. This takes only a few minutes to dry, before painting it, spraying it with sealant and peeling it off. The latex will form a mask of your sculpture. These are quick and easy to make at home.

Professional masks are a little more complex. First, you create an impression of your core sculpture, using either fibreglass resin or plaster of Paris, which is broken into two halves to create a mould. You then sandwich the core between these two negative moulds and pour in liquid latex. This is much better than the simpler process decribed above for two reasons. First, by squeezing the latex, a mould creates a better shape and fit; and second, they can be used multiple times, so you only have to create the mask design once.

Latex has its downsides, though. Some people are allergic to the natural rubber. Latex also becomes very sticky if it’s exposed to even a slight rise in temperature – such as being on a human face. This means manufacturers usually vulcanise latex by adding sulfur bridges to link the poly(isoprene) strands. But while this prevents the latex from becoming tacky rapidly, once set it doesn’t move much – instead, you’re left with a static, expressionless mask that doesn’t adjust to your face. Perhaps the best example is from the Halloween movies – serial killer Michael Myers wore a mask of Captain Kirk from Star Trek, bought from a supermarket, modified and painted white to create the creepy character.

High-end masks are made of an entirely different substance: silicone. Originally used in the movie industry for creepy monsters or realistic wounds, silicone masks are widely available today. Silicone is a synthetic material, made of a backbone of silicon and oxygen chains. These are highly temperature resistant and so don’t require vulcanisation. They are more viscoelastic once set too. Silicone is incredibly useful, and you can find it in cooking utensils, coatings, electrical insulation and computer keyboards (to enable the keys to ping back).

Silicone is more expensive than latex but the results are far more realistic, as the silicone will hug the wearer’s face, picking up expressions such as opening and closing the mouth or wiggling the eyebrows. With hair and makeup, it can even be used to look (vaguely) like someone else. In the latest Star Wars trilogy, Daisy Ridley’s stunt doubles wore masks of her to pull off some of the films’ biggest fights. The downside of all this movement, though, is that silicone tends to rip and tear. That’s why most silicone masks have a woven plastic mesh inside, which allows silicone’s natural stretchiness, while protecting the material.