Wednesday, 2 June 2010

Closer inspection of the kiln contents

I did as much glazing as I could last night, until I was ready to collapse into bed. I still have more to do today, but I'm right on schedule for set up tomorrow. :) I can fire this afternoon and the kiln can cool over night. My business cards still haven't arrived yet, which I'm not very happy about, but anyway... back to the pots.

When I got the tiles out of the kiln I took a closer look at them. Scott inspected the break for an air bubble, but just as Kim predicted, there was no evidence of an air bubble at all, it was a really clean break. What I did notice was a strange discolouration in the centre of all of the tiles.

I searched the net for this but couldn't find a thing. Then I remembered reading about carbon coring. I looked this up and it was described exactly as it appears on the tiles.

For those who haven't heard of carbon coring (as I hadn't), it occurs when the bisque firing is done too quickly (I wish!) or when the "reduced" atmosphere (as in reduced oxygen) is introduced too early in the firing, by putting the bungs in the holes too soon (likely). I placed the bungs in the holes at just over 300c, and the other pieces seem fine, but as these tiles were piled up on top of each other it would have been harder for the carbon to evaporate. Carbon coring can weaken the piece as well, so maybe this contributed to the breakage?

Other pieces that weren't as perfect as they seemed at first were the pitchers/jugs. They both had very fine cracks in them along the crease where the pitcher starts to bowl.

These were only surface cracks, they didn't go all the way through, but I thought they might still be visible if I glazed them with clear glaze as I had planned. Thankfully I had a tub of wax resist and a big bottle of white glaze!! So I painted over the image with the wax resist and then glazed the rest of the piece in white.

Another thing I will have to remember for next time is to be careful of contamination. After painting the squirrel in brown slip I must have handled it with brown clay on my hands. Although not visable before the firing, when I took him out of the kiln he had dark brown finger prints on him!! I've glazed him anyway, and I'll just have to see how he comes out. It's pretty much the same for the whole kiln load though isn't it? As Whitney Smith says: "Clay is one thing. Glaze is another. Combining the two is a path taken by crazy people." I suppose I still won't really know if I have a show or not until tomorrow!!


Kim Hines said...

future reference, there are things you can do to repair those things before you glaze your pots. low fire magic mender will fix those cracks right up. there are other brands/products too so get whatever your supplier carries. the finger prints you may have been able to sand off. i know you're in a time crunch though.

the carbon whatever it's called (taught myself but neglected the technical terms lol) was likely caused because the tiles were stacked so high, that's what caused the localized reduced area. plus when the tile tried to shrink it couldn't, was under pressure when it cracked and blew itself apart. that's why you found them spread so far. baby's having a screaming fit... :)

Undaunted said...

Kim, you are absolutely amazing! You have given me so much advice and support recently. I really appreciate it. Thank you so much :)

I shall remember the magic mender :)

Kathy L said...

Don't be hard yourself Linda...its all a learning curve and one day you will look back on all this and smile. Soom you will be just like kim, helping other aspiring potters! I know you can do it!

Undaunted said...

Thank you kathy. That's exactly how I'm viewing it - as a learning curve. I know I will keep learning with every piece and every firing. Already I'm starting to understand some things that happened in the class kiln that the tutor was unable to explain!