Mold making
The first thing I need to make clear is that there are many different types of molds and mold making materials. You need to keep that in mind when you read this information since techniques that work well when making a "slip" mold may not work well when making a mold designed for some other application such as casting fiberglass or foam latex.
Lets start by understanding what it is we hope to achieve with our sculpture and mold; The sculpture is, of course, a model that we will reproduce as a latex mask with the help of a mold. The unpainted latex mask can be called a latex slip since it is the result of "slip casting."
Slip casting: A latex slip is achieved the same way a ceramic slip is achieved (if you know anything about pottery and ceramics.). The latex is poured into a porous plaster mold. The plaster will absorb the water out of the latex that it comes in contact with. As the latex begins to lose it's water content it will begin to thicken and form a skin on the inside of the mold. The more water that is absorbed from the latex into the plaster, the thicker the latex skin will become. This latex skin is known as the 'Latex slip"
Alright, now that you have that bit of knowledge we can talk about
the mold. The mold has to be porous so that it can absorb the water from
the latex. There are many kinds of mold making materials available but
you must choose the right one for your purposes. Typically, if you want to make
slip molded masks you will want to use a plaster product. There are different
types of plaster but as a general rule, they all produce porous molds. Some
of these plaster products are much stronger than others so you want to pick
a strong plaster. You also want to pick a plaster that does not have any
kind of sealer built into it that is designed to eliminate the porous quality
of the material. When ever I make a slip latex mold I use #1 pottery plaster
or regular pottery plaster. They are both pretty much the same as plaster of
paris that you can buy at any hardware or paint store. You can buy pottery
plaster from a US Gypsum distributor or a good ceramic shop. Here is a
data sheet from US gypsum that give some specifications on some of their plaster
materials.
Pottery plaster is available in 100 pound bags for around $15.00.
Finally!!, Lets finish this mold.....
To make this mold I needed around 25 pounds of pottery plaster although the mold ended up weighing around 10 pounds... Its always better to have more than you need than not enough... I also used a few pounds of water clay to build the flange line.. I'll explain....
Here you can see the sculpture with a dividing wall built up around it. That wall is the flange line and it's purpose it to provide a boundary so that you can make half of the mold without having the plaster run all over the other half. Since I wanted the mold to be an inch and a half thick, I made the flange line an inch and a half high. You will notice if you look closely at the water clay that there are spherical depressions in the surface. These will become keys so that you can close the mold accurately when it is complete and you are putting the two halves back together...
Once the flange line is set up, you can mix your plaster and apply it to the sculpture.. Remember that plaster sets up pretty quickly so work fast but try not to trap air on the surface of your sculpture. I usually brush the plaster on with a 1 inch wide brush. You want to try to get atleast a half inch thick build up of plaster on the front half of your sculpture. Make sure you also brush plaster onto the flange line. When the plaster is about a half inch thick, you need to let it completely set until it is completely hard (at least 4 hours) .. The reason is that plaster expands when it sets. Remember, this is just your 1st layer of plaster. You have to put more on top of it... If you don't let the first layer set up completely, the expansion of the second layer will cause the first layer to crack since it will not yet be strong enough to withstand the forces of the 2nd layer expanding... So, why don't we just put all of the plaster on at one time? Well, you can but like I said, plaster sets up quickly and you may not have time to put it all on at once. Plus, the expansion of the plaster seems to be proportional to the thickness of the mold... I've made 1 inch thick plaster molds with a single batch of plaster and it expands so much that you can actually see areas where the mold is not touching the clay sculpture... Best thing for you to do is experiment... Get use to the plaster and the setting time.. Remember, cold water will give you extra working time and warm water will make the plaster set up quickly..
Once the front half of the mold is complete, you can remove the water clay. When you remove it you will see how the spherical depressions that were in the water clay have formed half sphere protrusions on the flange. These are the keys that will "lock" into the back half of the mold once you make it....
Before you make the back half of the mold you will want to apply a seperating agent to the flange. This is so the back half of the mold does not stick to the front half. I usually use petroleum jelly that has been thinned down with naphtha (a paint thinner) or sometimes I use a spray on release, which is basically just petroleum jelly is a spray can... Make sure you cover the entire flange but try not to get any on your sculpture. You don't need a thick layer. The thinner the better. Make sure you didn't leave any gobs of petroleum jelly. All you really need is a thin residue.... You'll notice in the picture below that there is a line of water clay still present along the top edge of the flange on the front half of the mold... I usually do this just so I can easily locate where the two halves of the mold meet when the time comes to open it... The plaster for the back half of the mold is applied just the same as it was for the front half.. Once the back half is set up (around 6 hours minimum) You can use thin wooden wedges (popsicle sticks) and a hammer to open the mold.. You will have to taper the wedge so that it can make it's way in between the two halves of the mold.. Once you locate the flange line (where the two halves meet) start opening the mold with the wedges. Important not to try to open the mold from just one spot, it might crack. Uses wedges all along the complete length of the flange until the mold is open and you are able to take it off of your sculpture. Don't be surprised if the mold does not want to let go of the sculpture. You may end up digging the clay out by hand..
More
to come later........