Porcelain Pigs in a Row
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The uniformity of these Pigs in a Row - made from English porcelain clay - is made possible by using a technique known as press molding. It is one of the oldest methods of mass production and goes back thousands of years.
In press molding you need (obviously) a mold. But just any mold will not do. You need a mold of a rigid material that will absorb water but will retain it's strength. Today, as in the past, plaster is the material of choice.
The problem with plaster molds is you have considerably more difficulties than in making and using a rubber urethane or silicone molds which today are preferred for making wax models for the lost wax method which is favored for bronze casting. Specifically, if you have a (reasonably) complex figure like a smiling pig, you need to make a piece mold. That is you need a mold made of a number of separate pieces that fit together like a three dimensional jigsaw puzzle. Unlike rubber, plaster has no flexibility, and the mold must be fashioned so there are no undercuts that would lock the mold to the original model.
Even without going into great detail, the process still sounds complicated. You take your model and define regions that when covered with plaster will avoid undercuts. Then you pick the part of the model to make the first piece of the mold - usually at the bottom - and with oil based clay form dams and shims at the border of a selected region so that when you pour in the liquid plaster it will solidify to form that particular the piece of the mold. The plaster will be liquid at first and so the details of the model will be rendered. Once this piece has set you remove the clay shims and put up new ones to make the next piece usually adjacent to the piece just made. Of course you do use mold release where the liquid plaster comes into contact with the plaster pieces that previously have solidified.
As you make the pieces it is important they have "keys" - that is depressions and protuberances in the proper places (no wisecracks, please!) - so on the individual parts of the mold fit together. You then continue to create the other pieces of the mold until the entire mold has been made. The number of pieces varies depending on the complexity of the model. Simple models might only take two pieces while complex sculptures with lots of undercuts can take a hundred or more. These pigs were made using a ten part mold - and that really wasn't enough.
But if you do it right, you will end up with a plaster mold that will come apart properly in the individual pieces and can be reassembled. Once the plaster mold is prepared an outer mother mold is usually made to hold the pieces together. Then you let the plaster mold pieces dry completely so no free water remains in the mold.
Now as the psychiatrist in Portnoy's Complaint said, "Now we may begin, yes?"
It is, of course, advisable not to make a mold of any kind without proper instruction. And it is imperative that you do not attempt to learn using models that are valuable or not easily replaceable. In fact, no model that you want to keep as a work in it's own right should be used unless you really know what you're doing. Models used to make molds - or flexible molds - often end up the worse for wear if not totally destroyed.
Compared to making the mold, creating the actual sculpture is relatively simple. First - and this is important - you add a light amount of talcum powder to the inside of the mold. This acts as the mold release. You do not want to use the usual mold releases like silicone or vegetable oils since these substances inhibit the absorption of water of the clay into the mold.
Next, you begin the "pressing". That is you take a small piece of your water based clay and press it into the inside of the mold, firmly enough so that the clay is spread to a thickness of about 1/8 of an inch. You then take another piece of clay and press it into the mold so it overlaps with the first piece. As you press the second piece of clay into the mold, you make sure to the push it into the first bit of clay so that there is no visible border. You continue to add clay in this manner until the inside of that part of the mold is covered. You usually will want to then add more clay to get a suitable overall thickness piece. A good thickness for a fairly small piece like these is about 3/8 of an inch. Next you proceed to the next piece of the mold.
When the inside of all the pieces are coated with clay you then assemble the mold. When you do this, you press the clay together at the joins (yes, the word is "join" not "joint") so the individual clay pieces merge together. There's no hard and fast rule for how you do the final pressing and assembling. Sometimes you can assemble the pieces of the mold first and join the clay together. In other cases you may have to seal the joins as you assemble the mold. In the end, you should have a clay sculpture inside the mold.
Now you just let the mold sit. This is to allow the water to absorb into the plaster partly drying the clay. Ideally the drying will produce what is called the "leather" stage of clay, which is firm, but not totally dry, and where the clay has some give. The amount of time the sculpture sits in the mold varies, but some books suggest three days. For these sculptures, though, the molds were disassembled after a few hours or overnight.
So if all goes well - and you have a good mold - you will have a hollow clay copy of the original model. Excess clay is then trimmed off at the joins, and any parts touched up that need it. At this point the process is no different than making any other fired clay sculpture. You let the sculpture air dry and then have it fired and then if desired patinaed or glazed. Honesty compels CooperToons to say his mold is not a very good one and considerable retouching was required. But all in all using the piece mold saves time if you want to make multiple clay sculptures.
Piece molding is not to be confused with a similar method of ceramic reproduction known as slip casting. In slip casting you pour a very thin clay/water mixture - that is, "slip" - into the mold. You slush it around to coat the inside of the mold. The water is again absorbed into the plaster and you then add more slip until you build up to the thickness you want. Slip molding can reproduce very fine detail and the sculpture can have thinner walls than a sculpture made from press molding. Slip molding, though, is not for the fainthearted and requires considerable expertise.