How does sintering work?
Clay consists of many different ingredients: quartz, feldspar, ilmenite and hydrated iron compounds, silica and aluminum salts … Before the clay has been fired, there is still some air between all of these particles. There are tiny pores in the clay, so to speak, and as a result the pot is currently still porous .
Once the kiln reaches a certain temperature, the clay particles are chemically activated and clump together. The pores close and the loose particles melt together into one solid whole. This is called sintering. Because the empty space between the clay particles disappears, your pot will shrink slightly during sintering. But it is thanks to this sintering process that your pot comes out of the kiln a lot stronger and more durable.
What can go wrong during sintering?
Without sintering, your pot would be as fragile as it was when it went into the kiln. It’s a crucial step. So if something goes wrong during this step, there are huge consequences: cracks, holes, unevenness, or even a broken pot.
- If the temperature in the kiln is not uniform, different parts of your pot will sinter at different speeds. This means that the clay will shrink at different speeds, resulting in cracks.
- But even if the temperature within the kiln is uniform, the surface layers of your pot will always heat up a little faster than the deeper layers of clay. That’s not a problem if it happens gradually, but if the temperature goes up too fast, the difference between those layers may become too great. That’s exactly why firing pottery is done slowly, in a very controlled way.
- There are always small inconsistencies in clay: here and there the clay particles are more close together. That’s exactly why kneading is so important: you make sure the clay is uniform throughout. If this is was not done properly, the clay particles will sinter at different speeds, resulting in cracks, holes, and unevenness.
