Learning to Lay Up Alginate and Make Hollow Casts

In the search to make bigger and better casts, John Gonzalez and I decided to work towards large scale body casts. As a goal, we first set casting my wife's pregnant belly. But before we could attempt such a thing, we would have to experiment with alginates and casting materials to develop new mold making and casting skills that would be required for this process. First, we would have to abandon the immersion mold, since this is not practical at the scale of a human torso. We would also have to abandon the solid pour technique for casting. The forms are simply too large for a solid plaster cast by measure of weight alone.

If John and I were going to be able to make a body cast, we would have to learn how to lay alginate on a vertical surface. So we met to accomplish this next step required to advance our skills. John volunteered his back for the effort. We had just acquired two new working materials: Hydrocal FGR (a fiberglass-reinforced gypsum) to be used in lieu of Plaster of Paris, since it is lighter and stronger; and an alginate that has been mixed with tiny fibers, so that it holds together well when working in the vertical plane. The alginate was a little harder to mix, but it did go on thick and with relatively little drip. The working time is a bit short at five minutes, and we were just able to cover his back in the time allowed.

We then proceeded without delay to make a mother mold out of plaster bandages. This rigid shell would support the alginate mold when we removed it from John's body. Since the alginate is flexible like a rubber, this support shell must be built to support the shape of the mold for casting. As we removed this shell, the alginate and the shell came apart, but we were able to fit them back together before casting.

The mother mold came off first, and then we peeled the alginate away, and fit it back into the rigid shell. We then placed this flat on our work table and started pouring the Hydrocal into the mold. Making a solid pour would not work, since the plaster would simply flow out of the mold. But we also wanted a light casting, so we would try to work the plaster into the mold surface, and hope that as it set we would have the ability to really produce a cast that was strong and of uniform thickness. But the Hydrocal refused to set in what seemed to be a reasonable time frame. Clearly our mix ratio was off, but we also had to realize that this stuff just behaves differently from Plaster of Paris. In the end we had a nice rigid shell, but the surface was quite cracked from the poor Hydrocal mix. While "painting" (or rather 'laying up') plaster into the open mold did work, we have a long way to go with this material to get it to a working consistency that we can truly control.

There were many successes here.  We made a successful mold and a hollow cast that reproduced enormous detail like the wrinkles in John's skin between his shoulder blades (above), and the pores in his skin (below).  

The casting was brittle, but next time we would be better with the plaster mix. The shell was cast hollow, so we know now that this could be done.  My fingers in the picture below show the thickness of the shell.  Pretty good, and essential if we are going to try to cast a larger area of the body and keep it light.  But most importantly we emerged from this experience knowing that a few more skill sets would have to be acquired:

1. We would have to learn how to get more coverage out of the alginate.
2. We would have to find a way to adhere the mother mold to the alginate, since the slip on removal led to a bit of deformation from the mother mold.
3. We would have to get better control over the plaster during casting.