Friday, April 20, 2012
Last night was the first firing with the new compression fit lid for my L&L DaVinci. Success! Before starting the kiln, the lid was sagging less than 1/8 inch in the middle. When I checked it at 1500 degrees, it was only down to 1/4 inch! This is much less sag than the original mortared lid had, so I'm very pleased so far. This morning I checked it again when it was down to 800 degrees, and it had only come back up a hair, so it may just stay there. So now I'm wondering if it will just stay put in future, or get lower with each firing. I do expect more sag in a glaze firing because of the higher temperature. Even if it starts sagging more than I'd like, I can always loosen up the clamps and adjust it back up. The bricks all look good, with no signs of crumbling. I'll be firing another bisque in two days, then a glaze firing the day after that. Fingers crossed!
Tuesday, April 17, 2012
My largest kiln is an L&L Davinci T3427-D. It is a 20 cubic foot top loading kiln, 46 inches wide, 32 inches across and 27 inches tall (interior dimensions). It holds three 14 x 28 inch shelves per row. Due to its large size, the flat lid tends to flex a lot, and the total lifespan is not as long as on the smaller kilns. It's just a really big span for a lid held together with mortar. Seeing the lid flex and move got me thinking about other designs that might work.
Knowing that it had to be a flat lid, I immediately thought of the Minnesota Flat Top, a type of gas kiln which has a flat slab top held together by compression, instead of an arch. Last year I started talking with Stephen Lewicki, the owner of L&L, about the possibility of using the MFT design on the DaVinci. After tossing some ideas back and forth for several months, Stephen had his guys build a prototype, which I installed on my kiln today.
The new design uses compression, rather than mortar, to hold it together. The hope is that without any mortar joints to fail, it will last a really long time. The compression is provided by threaded rods with a bracket at each corner. When tightened, compression is applied in all directions, resulting in a very sturdy slab. There is almost no flex whatsoever when raising and lowering the lid.
We were able to use the existing hinge system, so no modifications had to be made to the original kiln.
The weight of the new lid seems to be about the same as the old one, possibly even a bit lighter, so the existing springs in the hinge did not have to be replaced.
I'll be doing the first firing with the new lid later this week, a bisque. I'll be putting a set of shelves up at the top of the kiln to support the lid should anything go terribly wrong, but I really don't expect any problems. Here's what I'll be watching for during the next few months of firing:
1. How much does the lid move during firings? I'm expecting it to flex downward as the hot face expands. Will it return to its original position as it cools, or will the bricks shift? If it becomes a problem, I can loosen the brackets and raise up the bricks in the middle of the lid, therefore having a slight hump in the middle to compensate for the sagging. It's a very adaptable system!
2. Will the bricks crumble as they expand, or will the metal frame flex enough to allow them to move? If they cannot move, Stephen said there is a type of high pressure bushing we can use on each corner to allow for the expansion.
3. How much will the bricks flake onto the pots as they move, if at all? Mortared lids don't generally flake, but if it's a problem, I can put a layer of old shelves at the top of the kiln to protect the pots.
Because the system is not mortared together, I feel good about being able to deal with any issues that might arise with this new lid system. It's totally adjustable. And if it just doesn't work out, then I'll put the old lid back on!