Shiny, pearl-like spots or discolorations on your porcelain?

I was talking with a friend the other day and she mentioned she had been getting some unusual discolored spots on her porcelain and couldn’t figure out what was going on. She described them as being roughly the size of a thumb print with a streaky area running downward from the bottom, and there was always just one on a piece, and it was usually on the front of the doll legs in the area of the knee. She had no idea what might be causing it, and was frustrated because of the number of new legs she was having to pour, clean and fire. Have any of you experienced this problem?

It reminded me of an issue I had with my kiln when it was new. I have a Skutt 8/18 that was given to me for Christmas by my mom the year I began to make dolls. Since it was going to be used inside the house in the finished basement, she also bought the Envirovent to go with it. My husband followed the instructions for the Envirovent, which included drilling a certain size hole through the center of the kiln lid. It seemed to work fine. It was vented to the outside to get rid of fumes that are produced when firing, and it didn’t heat up the house.

I did however begin to notice that if I placed pieces I was bisque firing under the hole in the lid, I would have a spot on the finished piece that was hard to paint. It just wouldn’t stay on. Closer examination revealed an area that had a different texture, and usually looked somewhat streaked. With experimentation, I discovered that this problem was the result of air being sucked in through the hole in the kiln lid and blasting across whatever doll part I had placed there. I soon learned not to place anything in that spot when I was doing a bisque firing.

But after seeing a piece of my friends porcelain with the offending spot, it was apparent that this was a different issue altogether. These spots were clearly visible immediately, no painting involved. They didn’t respond to being re-fired like an under-fired area would, they just remained the same. And it didn’t seem to matter where the pieces were placed in the kiln. Clues that maybe the problem started before the firing?

The molds being used had been purchased new, and she said she never touches the insides of the molds with her fingers since she knows that the oils from her fingers can interfere with moisture from the clay being absorbed properly, and it’s really easy to accidently leave fingernail scratches. She cleans the inside of her molds with a soft brush before pouring. So that seemed to leave the method of pouring as the possible culprit.

I was taught to pour in such a manner that the porcelain first hits the edge of the pouring hole of the mold, as I’m sure you were. This helps to get rid of any small air bubbles that may have formed after straining the clay into your pitcher, and helps prevent wear on the mold where the clay would otherwise hit when poured directly down into the mold. As we talked about it, she realized that she hadn’t been following this rule consistently, and it makes sense that when pouring legs, the shape of the area around the knee would cause the porcelain to hit in that area as often as not.

My friend did some experimenting of her own, and sure enough, she didn’t have any more trouble with the spots when she made sure she was pouring onto the pour spout and letting the slip run down from there.

Maybe you knew right away what the problem was, but I’d never encountered it before, probably more by luck then anything, but now that I know what can happen, I’ll be sure to watch how I do my pouring. If any of you who are new to doll making are having this same problem, this may well be the answer for you.

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