Crack Para Mdt 6 BETTER
Crack Para Mdt 6
Burn any visual cracks as seen here. Ceramics crack during drying after glazing, but they dont crack while firing. This is a vast difference, and it gets easier to see each time (as the crack is being filled). Ceramics traditionally crack due to unequal shrinkage. With earthenware, it is most likely due to thermal stress. We typically fire ceramic pieces under air (without oxygen), this airless burning is closer to what happens in a kiln. The reason for this is that when we fire ceramics in ambient air, oxygen in the air reacts with the ceramic and makes it harder. If airtightness is not maintained while firing, you can see this through internal porosity. The more oxygen that reacts, the more rock hard and brittle the piece becomes, and the quicker the crack will go.
The more pronounced microstructure usually makes the cracking. We all know that high angle grain boundaries are more prone to craze. But a more even porosity is also useful. The surface of the piece should be a relatively smooth and pleasant smooth flat. We tend to get rid of air pockets while glazing, but still after kiln firing. With a clean bottom, air pockets shouldnt appear there. When glazed, we need to be sure the entire piece is covered by glaze (oven glaze isnt as strong as final glaze). A layer of glaze on the bottom means extra stress. Glaze-up of the bottom should be very even to minimise these wrinkles.
What makes that crack? The outside edge of the mouth of the kiln. Seems obvious, right? Well, no. Not always. There is a fundamental flaw in ceramic glazing : the outer mouth of a kiln is too long. It is a study in frustration when trying to glaze large ceramics. The large pieces are prone to sticking on the lip of the kiln. It happens with glaze, and when the piece comes out, there is a sweet spot at the time when it doesnt burn in place (about 35 seconds). If it isnt really stuck to the kiln, it will burn out at that time. But if it is really sticky, it will stick for a few seconds longer, and then burn the edge. This is not a problem if the piece is made of symmetrical pieces, but usually large glazing pieces are not (usually the opposite is the case). You cant really use the kiln until its burned, so the burnt edge is a problem. A kiln is a useful tool in the studio, but we can make the kiln do what we want it to do, and it has a huge role to play in the studio. It is a fine tool and should be safe for the studio. But some precautions should be taken. First, wait till the kiln is empty before you open it. Second, make sure you have a kiln cleanup cloth. Third, make sure the glaze isnt sticking to the mouth. Too bad these arent really useful tips.