↞BACK
This tutorial outlines a method for making a single-use, plaster mold that will be used to make a concrete casting.
If you work with plaster enough, you'll likely develop a selection of tools that you routinely use. Everything shown in this image (except the calculator) I keep together in the same place so it will be "ready-to-go" whenever the need to pour a little plaster arises.
The easiest way to create a plaster mold is to pour the plaster in a manner similar to the approach one might take when making a plaster blank (i.e. in the case of making plaster models). If you don't have a set of flexible forms, one can use either strips of aluminum flashing or simply use wooden slats to create a square form (this will, though, use a little more plaster). I use clay to hold the form in place and to provide a seal against leaky plaster.
While purists may frown, my approach to things is to measure all quantities volumetrically. It's just easier. In this image is a 9-ounce cup being used to measure out some plaster.
For this project, calculations proceeded as follows:
1) volume of hardened plaster = π/4×12²×¾ = 85 inches³ (diameter is 12", thickness is ¾")
2) volume of mixing water required = 85×12.3 = 1045 cm³
3) or 1045÷30 = 35 ounces (this is about four 9-oz cups of water)
4) to this amount of mixing water I will add six 9-oz cups of dry plaster
After mixing up the plaster, slowly pour it over and around the clay model. If you add it slowly enough, it will penetrate all the details (e.g. the eye pupils) without leaving behind a lot of air bubbles (air bubbles, if they form, can be dealt with later, but it's always easier to avoid them).
We mixed up enough plaster here to cover most of the model with a thick coating that should be strong enough to withstand the process of casting (another tutorial), but the nose only received a relatively thin cover. Not to worry. We'll take care of this in the next step.
After the plaster sets, you can mix up a small amount of additional plaster and simply coat the nose with enough to give it a coating of something in the neighborhood of ¼+ inch.
After all the plaster has set, we're now ready to remove the clay from the mold. To accomplish this, I use a set of tools that allow one to more or less "carve" out strips of clay from the model. I made my own using a spring (it is very stiff wire), which was hammered into the shape I wanted, then epoxied into some small lengths of PVC pipe. You can also purchase tools like this from any pottery supply shop.
Start carving out the clay. I try to stay away from the edges to avoid scoring the plaster inadvertently.
After removing enough clay with the carving tool, you will likely be surprised at how easily relatively large chunks can be removed. You can also use a knife blade to "pry" clay away from the mold for easier removal.
When you get to the area surrounding the eyes, slow down and be careful. It's very easy to get in a hurry and damage a part of the mold that, while repairable, would be a hassle to do so. Also, there are likely areas where the plaster has "penetrated" into the clay further than what you might have anticipated. Just break these pieces off. They are unwanted "undercuts".
After removing all the clay, place the mold in front of a bright light and look for thin spots in the plaster. These will need to be strengthened with some additional plaster.
Mark any thin spots in the mold with a pencil.
Mix up some plaster and cover the thin spots. It doesn't take a lot. Just add enough so that the areas "pass" the "lamp-test".
After removing the clay, the mold is likely to look a little "rough". Not to worry. "Ratty" looking edges, air bubbles or creases are quite easy to touch up and smooth out.
Methodically work the mold removing sharp or feathered edges of plaster. You'll want to do this not only around the perimeter but within the interior of the mold as well. When it comes time to remove the plaster from the eventual casting, not having thin, difficult to remove, sections of plaster will make the job easier.
Some pieces you can simply break off with your fingers.
One can even use one's fingernail like a blunt knife edge. As a general rule, sharp plaster edges can be easily rounded and blunted by simple finger pressure.
The one place where you may want to be careful and approach things in a more exact and painstaking manner is around the eyes. I use a sharp knife and carefully taper the areas around the eye balls and pupils. Leaving undercuts in place is O.K. if that's what you prefer. They're just more difficult to work with, both in terms of making the casting and then later removing the mold.
Air bubbles can be fixed using the "layering" technique outlined in a tutorial else on this website.
After touching up the interior (e.g. rounding feathered edges, removing "undercuts", filling air bubbles, etc.), set the mold aside to dry. Applying the mold release I use (bar soap) works best with a dry mold. Waiting for the mold to dry also makes it a little easier to remove whatever "crumbs" might be left behind by the touch-up process.
