I fire my kiln manually – I’m too poor for any fancy electronic controllers. I have got used to a slow heat up until all moisture is gone, then pretty much push it to the max until the kiln sitter drops.

Pinhole in ophthalmology

I had a very unhappy kiln opening a few weeks ago with glaze dimples and pin holes all over my bigger pieces. I’ve had the problem on a smaller scale before, but nothing to this extent. It’s quite soul destroying finding a load of work that needs to be binned. I sulked for a couple of days, then with gritty determination I vowed to find a solution!

Previously I have just fired straight up until the cones drop, then turn off the kiln. I think as I have a very small kiln, the glazes are not able to develop properly with this fast firing style.

Pinholesin paint

If I have no luck with that I have also bought a different clay to try. It’s an off-white stoneware with no speckles. It will probably effect the appearance of the glaze but the colours could be adjusted if necessary.

It seems the lack of carbon burnout was causing the problems with glaze pinholing in my case. I would say there were a number of contributing factors in my case.

How to fixpinholes

I’m pretty sure the kiln vent is working as intended due to my flame tests. Even if it isn’t, the few pots I bisc fired must have had plenty of oxygen round them.

I read an article about using a longer firing cycle to avoid pinholes. For my last firing I held @ 1120C for 30 minutes, then climbed slowly until cone 8 was down. Then I dropped the temperature to around 1230C and held for 20 minutes before turning off the gas and closing the kiln up.

I hadn’t meant to apply less glaze to the bigger items. During dipping I’ve keep the time in glaze the same as for smaller items. I should really dip the larger ones a bit longer as they take more time for a good layer to adhere.

Initially, as a glaze defect I began pointing fingers at the glaze. The pieces in question have been glazed in two different layered glazes. This is a new technique for me and it was quite possibly the culprit. The only problem was many of the smaller items in the same load had the same glaze and had come out fine.

Pin holes defect

Pinholesmeaning

I didn't want to change my clay just because of glaze defects -  I like the look of speckled clay. No matter what I've tried to adjust my glazes I have always struggled with pinholes. They problem is often worst on larger pieces such as plates and vases.

I’ve decided to try modify the glaze with additions of extra frit in varying amounts to each layer. Hopefully a lower melting glaze might seal up the pinholes and smooth out the orange peel surface a little.

Pinhole glasses

Unfortunately even after all my best efforts on the carbon burnout front, the resulting plates were almost exactly the same as the previous batch.

I wrote most of this article while the kiln was firing. I hoped to add a successful conclusion at the end and live happily ever after!

Pinhole camera

The factors linking the most problematic pinholed pieces was their size, and the fact I stacked them during the bisc. It was my nine inch pasta bowls and plates. I had used sand to prevent stress on the rims but they were very densely packed. The more I read, the more it seemed the bisc firing was the problem.

Next I tried to blame the clay. I use a heavily speckled stoneware which is said to be more troublesome than purer clay mixes. But again, the smaller pieces made from the same clay had turned out fine.

I have taken a closer look at all the pots using this particular glaze. On the whole the bigger items have more pinholes. On the smaller olive dishes it seems the ones with more glaze have no pinholes but a slightly orange peel type surface. This would suggest the glaze may be a bit under fired.

Edit: I wrote the rest of this article a while ago. Since completing it I continued to have problems with pinholes, although switching to a white clay without speckles did help somewhat. I'm now firing in a gas kiln.

Out of the thirty or so pots from my last firing none had any noticeable pinholing. This was using glazes that have had problems with it before. Fingers crossed I continue to have success in the future.