• Clay shrinkage

    I’m going to have to figure out what the clay shrinkage actually is and allow for TWO shrinkings to wind up with the size I want to do pressed clay versions of my work.

    I found the little cherub plaque really loses a lot of it’s visual impact due to the shrinkage. The original was 10″ wide and the fired terracotta is only 8-7/8″, that’s quite a reduction.
    So the way it has to work now is making a clay model large enough to compensate for it’s own shrinkage plus the shrinkage of pressed clay copies of it, I’m not great at math enough to easily calculate what this will be per foot, here’s some measurements I do have:

    Original: 10″
    Shrinkage: 8-7/8″

    Original: 9-1/2″
    Shrinkage: 8-1/2″

    Original: 4-5/8″
    Shrinkage: 4-1/4″

    Best I can come up with is the clay shrinks one inch on a 9-1/2″ original, so if I take 9-1/2″ and divide it into four it is 2-1/8″ which on a model that size would shrink 1/4″
    Adding that to the 9-1/2 would be 11-5/8″ and 1-1/4″ shrinkage, I guess for my purposes then I can assume about 1-3/8″ per foot for shrinkage.
    I would have to allow for two times that for a clay model plus the resulting pressed clay casts.

    Here’s a design I want to do:

    He measures 15″ high, 10-5/8″ wide at the top and 6-1/2″ at the bottom,
    but none of those measurements are 12″ or multiples of it, so I really kind of need to know what the mathematical formula is to figure out that 1-3/8″ shrink over 12″ what it is on 15″ or any other non 12/24/36/48 measurement.

    The 15″ is not too bad, as that’s 12″ plus one quarter of a foot ( 3″ )
    so I can figure 1-3/8″ + 1/4 of that but that still doesn’t teach me the FORMULA to do it easier. I come up with 2-1/8″ shrinkage on 15″ but double that means I’d have to make the original model 15″ tall plus 4-1/4″ to compensate for the shrinkage to allow the fired pressed casts to end up just about 15″ tall!

    In any case I managed with this one since 15″ is 1-1/4 feet so the measurements were not hard to come close to.
    I made my box form 19-1/2″ high inside, and 13-3/8″ wide at the top, 8-1/8″ at the bottom, 4-1/2″ deep and screwed it together.
    I have a curved template I used once before for the bottom of a keystone having a curvature for the top of an arched window, after I get the form filled with clay and levelled off 4-1/2″ thick, I take the bottom board off after laying the form flat on the bench, and then use the template to shape the clay, then reassemble the board and lean it back up on the easel to finish work on it.
    The form depth is the portion of the sculpture that would/could be embedded into a brick wall, so when I make the plaster piece mold I’ll have to make the four sides in such a way that they are in 8 pieces, so 4 of them made about 3″ deep can be removed and the clay pressing cut with a wire so this backing portion behind the actual face is only an inch thick so it can hang on the wall.
    This will be the first time I’ve done this but I already figured out the mold keys will have to be done so that the concave portions are on the backs of the narrower sides. The convex portions will have to be on the front edges of the 4 rear side pieces that get removed to make the casts thinner, that will avoid having “bumps” where the cutting wire would have to move flat against the mold.