A matrix mold appears like many other molds, where a thin mold of silicone rubber is supported by a thicker shell of plaster or fiberglass. However, you make a matrix mold by pouring the plaster first, then filling the area between the plaster and the model with silicone rubber. You do this by using clay to take the place of the rubber when pouring the plaster.
It is a very economical way of making a mold, since you use the least amount of silicone rubber necessary; the rubber is the most expensive material in a mold.
Matrix molding is one of the new techniques added to the second edition of The Prop Building Guidebook: For Film, Theater, and TV, which comes out February 10, 2017. You can see all the companion videos at the Prop Building Guidebook website.
I have a new video up on the videos page of The Prop Building Guidebook website. This one deals with making a one-piece plaster box mold. Plaster is a cheap and relatively easy-to-work-with molding and casting material, and a one-piece box mold is one of the most basic types of molds to make. I feel a one-piece plaster box mold is one of the best introductions to mold making for those reasons. If you can pull it off, you’ll have more success as you move on to silicone rubber and other fancy mold-making materials.
So check out the video below, and don’t forget that I’ll have new videos up every week until my book comes out on February 26th.
The following article was originally published in the New York Times, November 25, 1906:
Situated in the storehouse and wharf district of the extreme west side, and running through the entire block from Twenty-seventh to Twenty-eighth Street, the casual observer would never imagine by a glance at its unpretentious exterior that a veritable fairy-land lurked within its four plain brick walls.
The person who is fortunate enough to gain admittance past its argus-eyed German watchman stationed at the main door will be amply repaid for the visit. Erected by Henry W. Savage, a theatrical manager, for the building of his plays, the play factory is capable of turning out even the largest productions complete without their leaving the building for anything whatever, and here one may watch the entire construction of a play underneath one roof. The greater part of the time there are 250 people at work in the factory. These include scene builders, painters, electricians, costumers, florists, and property men. Continue reading A Factory for Making Plays→
Life casting a hand is one of the easier parts to do; the only tricky part here was that they were rehearsing uptown at Second Stage Theatre and I only had an hour, so I had to prep everything beforehand so I could be in and out like a ninja hand caster before anyone knew what had happened.
Jay O. Sanders molding his hand
I used alginate because it’s cheap, non-toxic, safe on the skin, and sets up quickly. It’s made from seaweed and used in dentistry to make molds of your teeth and gums. After mixing the powdered alginate with water and filling a hand-sized container, I asked Jay to pose his hand and submerge it. It doesn’t heat up like plaster; it just slowly thickens. In about ten minutes, it had solidified enough to where Jay could remove his hand. It’s stretchy enough that he could pull his hand out without breaking or tearing the mold (if done slowly), and it doesn’t really stick to anything (no mold release is needed). The little bit that does stick can be washed off with water.
Pouring plaster in the alginate mold
The thing with alginate is that it starts shrinking right away as it loses water. You only have about two or three hours to make a cast before it has shrunk noticeably. I mixed up some plaster while still at Second Stage and poured it in. After it had hardened enough, I packed up my stuff and took the subway back down to the Public Theater.
Plaster hand inside the alginate mold
Now I could just tear the alginate away until the plaster hand was free.
Brush-on mold
I decided to do a brush-on mold for this because of the shape of the hand. I had only attempted a brush-on mold once before and it didn’t go so well. This time around was only marginally better. I ended up making it work, but that’s about the only good thing I could say. After the rubber mold set, I built a two-piece plaster shell around it for support. I ended up having to cut open some of the silicone mold to remove the plaster hand inside.
Unpainted silicone rubber hand
I cast the hand itself out of Smooth-On Dragon Skin, which long-time readers may remember being used in both the deadbody and head for The Bacchae, as well as the deadlamb for Little Foxes. Dragon Skin itself cures fairly translucent and colorless, so I tinted the whole mix with a flesh color. I also embedded some dowels inside to cut down on the amount of silicone rubber I would need and to act as bones.
Hand of Titus Andronicus
I “painted” more Dragon Skin over top, tinted with different colors. I rubbed some darker colors into the cracks and crevices to give it a bit of depth, and added a bluish tint onto the veins. I was using a fairly thick mix so it wouldn’t run. Silicone rubber is self-leveling unless it is thickened, and it can be pretty glossy if allowed to do that. I didn’t want that on the hand, but I took advantage of that for the fingernails, where I used a pretty thin mix and only a touch of color to get a shiny and translucent effect.
"I kind of feel like that kid who found the severed hand..."
The hand has gone through some changes since I took these pictures; the bones on the back were cut down in length, and a lot more blood was painted on. All told, it only took about three days between them asking me to lend a hand making a cast of a hand to the cast having a hand in hand for rehearsal.
In Thurston James’ second book, he tackles the subject of molding and casting for prop makers in more detail. The Prop Builder’s Molding & Casting Handbook guides you through the most common materials and methods used in many prop shops. Because of its specific focus (and better organization), this book is far more successful than his previous Theatre Props Handbook, which, as I mentioned in my review, meandered through disparate topics with no way to quickly find information.
Though written in 1989, the methods described in this book still hold true today. Though the range of materials we can use today have grown dramatically, they remain improvements and new formulations to older materials whose predecessors can be found in this book.
It remains one of the most widely recommended books for molding and casting props because of the unique niche it fills. It describes the most common materials and methods used in props shops and by hobbyists; these materials are used because of their cost, ease of use, availability, and proven results. Books on molding and casting for manufacturing and industry are more focused on specific or specialized materials, and they aim for a level of consistency and cost efficiency which the prop artisan would never possibly need. Shaving a tenth of a cent off the cost of a casting makes a difference if you are casting ten thousand pieces, but it will be impossible to notice if you are only making ten.
James seems to have had an epiphany in shop safety between this book and the last, as he now presents clear and accurate safety precautions in the beginning of the book, and continues to reiterate them throughout. In his Theatre Props Handbook, safety precautions were nearly nonexistent.
The book does a good job of covering the generalities of mold making and casting. It discusses the model and its preparation, and defines a number of necessary terms, such as undercuts, release agents, mother molds and the like. It describes the considerations of making a mold of your specific piece, and breaks the various molding materials and casting agents into categories. In a way, it describes the process of choosing your materials in an almost flowchart-like manner. If you know what your model looks like, and you know what kind of properties and appearance your castings need, then you can narrow your choices of mold material and casting material down to a few choices. In the book, he describes over thirty of these material choices.
guides you through the most common materials and methods used in many prop shops. Because of its specific focus (and better organization), this book is far more successful than his previous 
, which, as I mentioned in