Tuesday Tips - Bisque Molds for Pendants

These butterfly pendants were formed from a bisque mold I made from a carved slab of clay. It's a technique I love because you can build up delicately raised texture for your glazes to pool and break over. Most of my designs start with an idea of how glazes will work to acheive the effects I want. But that's a whole other blog post. So I think we'll save that for later.

Most of my designs start with sketches. I wanted a final glazed pendant that would be about 1.25" square. So there are two things I need to remember before I start. First the bisque mold is going to shrink during the firing.  Second, my pendant is going to shrink as well between its raw clay form to it's final glazed form. How much will it shrink? Well that depends and it's really only possible to figure it out by doing a few shrinkage tests with your clay and your firing conditions.

But for me, I know that the stoneware clay I use for molds shrinks about 5% from raw clay to bisque fired piece. I also know that the porcelain I make pendants from shrinks about 15% between raw clay and glaze firing. From that I know that the bisque mold needs to be about 1.44 inches square to account for shrinkage of the porcelain.  To that I add another 5% to account for the shrinkage of the raw clay of the bisque mold.  That brings me to a 1.5 inch square starting point to size up my image.  Whew, that's a bit tedious, no?

So that drawing up there is a print out of a scan I made from the original. I just put a copy into power point, click on the image properties, and tell it to size it at 1.5" x 1.5" and easy peasy, I've got a properly sized drawing.

I roll out a slab that's a little over a quarter of an inch thick and let it set up to leather hard. I do this between two sheets of dry wall to keep the slab from warping. I find leather hard clay the best for carving because any softer and it's just too difficult to keep from gouging and tearing the clay.

The tools I use are pretty minimal, a wire loop tool and a soft brush to sweep the clay crumbs away. That image up there shows the design marked into the clay and the first start at carving. You know, you kind of have to think upside down when you do this because whatever I take away is going to end up raised on the final pendant.

A little further along in the carving. I take away clay quite slowly. You can always take away more clay but it's nearly impossible to add any back.

The completely dry slab of clay then gets a bisque firing. I prop mine up on end in the kiln. It helps to keep the mold from warping and even worse cracking. Sometimes large flat pieces will warp and crack if they are lying flat on a kiln shelf because the shelf temperature is hotter than the air on top.

Ta Da! My bisque mold and freshly pressed pendants.

One last little tip. By the time I got the lines of this particular pattern deep enough the clay was quite fragile. If I brushed away the clay crumbs I risked damaging the mold. So I just fired it and then used sand paper to smooth the surface of the little crumbs.

Come back next Tuesday and I'll show how these pendants get pressed. Thanks for stopping by!