One of the largest differences between wood firings and other forms of firing clay, mostly electric and gas, is the volatile and destructive nature of the firing process. The act of burning wood for fuel, releasing all the ash and carbon into the kiln, not only produces the beautiful and stunning results of a wood firing, but also causes a lot of damage to the pots, glazes, shelves and even the bricks inside the kiln. The very elements which we love so much in wood firings, the flames which path through the kiln and kiss the pots, and the ash which can build up and create its own natural glaze during the firing, also break away at the bricks, stick pots to shelves, and even sometimes leave debris or spots on pieces and ruin them. All of the pieces in the photo below are not up to our standards for one reason or another, and sadly will mostly be destroyed or put into the garden somewhere.
Each firing we expect to lose some pieces. Sadly and almost without fail, its usually our most treasured pieces going into the kiln which come out scared or damaged. Glazes oxidize when they should reduce, the temperature goes ever so slightly too high for too long and glazes run and stick to the shelves, or differences in glaze tensions occur, ripping and tearing the clay or creating irreparable x cracks right through the wall of a piece.
One of the most common “defects” from wood firings is crazing. Crazing is the term for the small, minute, spider like cracks and lines which form on the surface of a glaze. I say the surface because these imperfections are only skin deep and often not detrimental to the life of a functional pot … to a certain point :) Crazing is often desirable in certain situations and uses, and can create beautiful effects. Glazes which ‘crawl’ or use crackle effects take full advantage of this process to enhance a piece. Crazing in wood kilns is often something we cannot avoid. We make it a point to always glaze any food surfaces on our pieces with stable, glossy and safe glazes which are easy to clean and maintain. Glazes with very rough textures or matte finishes are actually hard to clean, can hold bacteria or dirt, and shouldn’t be used on a functional piece. Now, this opinion will differ between potters, and you will see potters (some of them our favorite and most respected potters!) glazing the inside of mugs and bowls with heavily crazed glazes because they love them. It’s a matter of opinion, but we try to err on the side of caution.
In the above two photo’s you can see some of the crazing which occurs during one of our latest firings. This could be the result of temperature issues during the firing (maybe not being hot enough), a slight imbalance in the glaze composition, too much of one chemical or another, or it can also be caused by wood ash falling on the pieces during the firing, a natural occurrence in almost all wood firings. We really strive to eliminate this effect as much as possible, but its almost impossible to avoid. Crazing will also increase over time as the glaze naturally matures on a piece and surface which looked perfectly smooth and uniform one day, will have visibly changed a few days later. We try our best to keep an eye on all our pieces and those with heavy crazing, we don’t sell. You’ll often find them in our cupboards and kitchen since they’re still great to use, but we feel bad putting out work that we know isn’t 100% perfect.
If you have a piece with a little crazing, enjoy it, it’s part of the natural process of wood fired handmade pottery, however if its really severe, you might have to be careful, and if its something we’ve overlooked, please let us know and we’ll be happy to replace it if we can.
X-cracks are cracks which damage the integrity of a pot beyond simple cosmetic damage and run right through the clay body. Often with gas and wood firings, there will be a certain amount of crazing on glazes. Sometimes its even by design to enhance or create a specific effect (crackle glazes are perfect examples of this and do not damage the integrity of the pot.)
However an x-crack on functional work usually symbolizes a real problem. They are often caused by weak points in the clay body itself, a point where during the firing, natural tension put on the pot was too much due to a weakness or overly thin area, and the piece breaks. It can also be caused by impurities in the clay itself, something that does happen since we recycle our clay and sometimes have small particles of debris in the clay when we throw. Also, different glazes move and react differently during a firing, how fast they heat up, how fast they melt and most importantly, how fast they cool down. If an exterior glaze cools down faster or slower than the glaze on the inside, a huge amount of stress is put onto the clay itself, and can break under the tension.
This is one of the problems which has frustrated and confounded my father probably the most, and we’re still scratching our heads trying to figure out exactly what we’re doing wrong. When glazes peel, they leave bald patches or spots on a piece where the glaze should have been, but refuses to stay. It’s a very odd phenomena because while many glazes run, they will still leave a thinner layer of glaze, whereas peeled glazes seem to leave nothing covering the naked pot where they should have been. Luckily for me, this seems to happen almost exclusively on my fathers pieces, and I seem to be happily immune to it most of the time. That leads me to believe it’s probably because of some combination of how he handles his pots when he’s glazing and cleaning them, or how he uses the oxides and mixes glazes leaves them vulnerable to this. Whatever the cause, it’s a problem which usually ruins an otherwise beautiful piece with a glaring blank spot left staring at you.
Running Glazes (Potters Tears)
Glazes are very temperamental, just like the potters who use them. Some are cool and laid back, stable glazes which hardly move, while others are hot headed and passionate, glazes which run and melt, sometimes right off the pot. This is a risk most potters manage by controlling the firing process as closely as possible. In electric firings, and most gas firings as well, you know exactly at what temperature your glazes will start to run, and you bring them just up to that point, but stay on the safe side. In wood firings, we’re terrible at maintaining a truly stable environment inside the kiln. All kinds of things are going on; stoking the kiln too quickly, using different types of wood (hardwood, softwood, wood with sap), how attentive you are, how much reduction is going on, how open or closed the damper is, we’re basically juggling a ton of things and if we get 8 out of 10 things right, we’re happy. And so, with our use of copper red glazes and wood ash glazes, we always run a high risk of having our glazes run. Sometimes the results can be amazing, sometimes we lose large batches of work, all at once.
These tumblers are an example of pieces I lost due to glazes running. Perhaps the cruelest part was that the glazes came out amazing and the copper reds were incredibly vibrant and beautiful. However, all of them ran, some sticking to the kiln shelves. I tried grinding the bottoms of the pieces to salvage them, but the glaze chips and breaks and leaves dangerously sharp edges. If anyone needs a tumbler to hold pencils or to put a plant into, please let me know, you’re welcome to take any one of these you’d like!
I do have to say that there is another form of firing, soda firings, which usually has an even higher loss rate. I’ve seen some potters open their kilns only to discover almost 80% of their pieces are broken or ruined. Thankfully we are nowhere near to those kinds of losses, barring something catastrophic happens during the firing.
Well, this ended up a lot longer than I originally had planned, but I definitely do have a tendency to run on and on on topics. I hope this was a little enlightening and helps to explain our love/hate relationship with wood firings and pottery in general. Pottery truly is a labour of love and when you see good, handmade pottery, its nice to appreciate the time and effort that went into creating such unique pieces. There is a lot of heartache and disappointment that goes on behind the scenes and knowing a little about it helps appreciate any craft or art form.
Thank you for your time,