This week two pieces of pottery were returned by a customer who wasn't happy with my work. One of the reasons she returned the pieces was her expectation that the bowl be fully glazed from the top to the very bottom of the piece, which is exactly what you find on factory-made pottery. I leave a 1/4" margin around the bottoms of all of my pieces to ensure that the glaze doesn't drip down and stick to the kiln shelf. I happily refunded her, but I thought that since she expected something different than what I made, perhaps some other pottery customers might also be surprised by the margin and bottom of unglazed clay and wonder why I leave the bottoms of my pots bare.
Pottery glazes have a very high silica content. When they're super-heated (2165 F, 1185 C), the silica melts and becomes glass. Because I dip my pottery, the glaze is somewhat thinner than brush-on glaze, and as it melts, it runs down the sides of the piece. This melting and running is also why some of my colors, particularly the robin's egg blue, can look streaky and uneven, rather than completely smooth and even.*
Prior to glazing, I paint a thin coat of wax on the bottom of my pottery so that glaze won't stick to it. I also carry that line of wax slightly up the side of my pot. Usually I go about 1/4", but if the piece has a distinct foot-ring, like on a berry bowl or a cafe au lait bowl, I leave the entire area unglazed. I do this so that even if the glaze drips down, it is likely to stop and not melt onto the kiln shelf.
Yesterday, still a quite floaty and contemplative from seeing the Dalai Lama's lecture, I took care of a few things around the house and went to unload the kiln. This load was quite full of re-glazes and I did not do a good job of re-waxing the bottoms and footrings of the re-fired work. At least 15 pieces dripped and adhered to the kiln shelf.**
This, predictably, made me feel not so floaty. These 15 pieces- four berry bowls, a several mugs, and several smaller bowls- represented a quarter of the kiln load that I can't sell. Four hours of prep work and glazing, three hours of throwing, trimming, and sanding. Because I didn't re-wax and watch those margins. I think that the berry bowls' dripping had something to do with the humidity (today is the 15th straight day of rain in Memphis). They were not entirely ruined because I can sand the bottoms give them to friends who won't care. But this otherwise lovely mug? It is headed for the trash. You can see spots in the top picture where the shelf/ kiln wash stuck to the bottom of the mug- and those shiny spots? That's glaze (I'd put a thin coat of clear glaze over the speckles on this mug- the speckles need to be covered with glaze to be food safe, so I do two glaze firings on each of these pieces). On the second picture, you can see where a chunk of the mug stayed on the shelf. This was a particularly painful lesson in not rushing and really inspecting my work before loading it in the kiln. I've been doing this for 9 years now. These look like mistakes of a first-year student to me. With this post, I'm releasing and moving onward.
*I have issues with this look- I'd prefer smooth and non-streaky, but this glaze doesn't behave that way, no matter what fixatives I add to it or how I apply it. Since it seems to be a popular color with local and online customers, I keep using it, but to be honest, the streakiness bugs me. I'm a reforming perfectionist.
**this also tells me that it is time to do some kiln and shelf maintenance. This coming week I'll don a mask and use a grinder to smooth off the shelves. Then I'll mix up some kiln wash (a powdered blend of silica, kaolin-clay-, and aluminum hydrate), brush it on and fire three times. My shelves are old and flaky, and they need some love. A proper coating of kiln wash ensures that your drippy pieces don't take a large hunk of shelf with them when you remove the piece.