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Showing posts from August 12, 2012

Third Mold

This mold was unfortunately unsuccessful, but I did learn a thing or two about metal. Ten minutes' bike from the farm is a forge and metal workshop, and one of the techniques they use is sand casting. A full-scale pattern is packed into sand, then removed, and molten aluminum is poured into the void. 

The pattern and the cast.

I made my pattern quite thin, thinking that metal would allow for an economy of material that wood does not permit. However, when the blacksmith examined my pattern, he remarked that the cast would be liable to break, because cast aluminum is very different from sheet aluminum and can be brittle. He said that it would be better if I brought a solid block with the hole cut out- exactly like First Mold!

The blacksmith examines my pattern.

The cast did not turn out because it was hard to remove from the sand. The sand must be damp in order to hold its shape, so the pattern must be 'smooth like glass'.

Sand Woes

Sand left in kiln after removal of bricks.

When I opened the kiln after my first firing, the bricks were...dark. Burgundy. Comparing this colour to the vibrant orange that I had seen before, I knew something was wrong. 
First, the sand I am using changes colour. It must contain a lot of iron, or some sort of clay-ish rock, because it turns from brown to burgundy when fired.
Second, I had fired the first batch to 1100C, whereas for every firing prior to that one I had only fired to 1000C. I think this changed something in the clay-sand mixture- maybe the sand melted at the higher temperature.

When I made test bricks with 50% grog, they were much stronger than just with clay. I thought that sand would work the same way, but turns out that sand makes the clay a lot weaker. The bricks from the first firing are very brittle and crumbly. I have changed the proportion of sand in my mix, so that it's now not more than 25%. I can't give up sand as a mold release, though, so I'll have t…

Notes on Consistency

Clay too wet- it coats my hands.

I have been struggling with the consistency of the clay. Mostly, it's too wet, which results in sticky fingers and misshapen bricks. Too dry is also a problem, since that makes it very hard to fill the mold, and leaves deeper creases on the surface. 

A further problem is that clay dries extremely unevenly. I can spread it on a board in the sun, and if I don't look after it every 15 minutes or so, the top develops a crust (too dry) underneath which the clay still retains all of its moisture (too wet).

Different batches require different working methods. For a wetter clay, I pound it into the mold with the side of my hand, because it's the only thing that doesn't immediately stick. With a dry clay, I use a rubber mallet, for more force. During demolding, I have to be very careful with a wet clay, because the brick tends to fold in the middle, so I have to tap harder on the corners to loosen it before letting it slide directly down. With a dr…