mold size?

[loricrawford]

I have another question. How much room should you leave around a mold? If the interior of my kiln is 11" wide by 9" deep what would be a safe size mold (largest) that I could use? Thank you, Lori

[Laurie Spray]

If your kiln is with the coil in the lid
you can use a mold that barely fits! If it is side fired you need around an inch.

[Morganica]

If your kiln is with the coil in the lid
you can use a mold that barely fits! If it is side fired you need around an inch.

Disagree--If you need an inch for side fire elements, why wouldn't you need at least that for top-fire elements? A "barely fits" (say, 8.5x10.5 inch) mold in a 9x11 inch kiln is going to be a headache, especially if it's only top or side-fire. You've got 6mm (1/4 inch) of glass on top of the mold, a quarter-inch from the elements. Having heat that close, with that little room for air circulation, is going to magnify every annealing issue, every difference in viscosity, etc. It's a little easier if you've got top and side heat, but I still would keep the glass farther from the elements.

Bullseye's got a nice technote about using their slumping molds (below), which recommends extra caution if you're firing within two inches of top elements. They also suggest elevating the mold to get better circulation and more even heating/cooling during firing. I'd agree with both of those things. I don't care if I'm within an inch of the top elements on the first firing with small components, but after that I'm dealing with a single piece of glass, usually with variable viscosities. Much trickier, especially when you start talking variable thicknesses, tackfuses, etc.--the stuff people usually want to try once they've done a few flat fuses.

So, using that as a guideline, you need to allow for 1.5-2.5 inches (1 inch from the top elements (and I'd rather have 2), and a half-inch of elevation off the floor). Doing the math, that means this kiln will conveniently accommodate molds of up to about 6.5/7.5x9 inches.

Doesn't mean you can't successfully use bigger molds in this kiln--you can--but you'll need to slow way down (longer firings=fewer firings/month) and accept a higher risk of loss.

https://www.bullseyeglass.com/images/st ... d_tips.pdf

[Laurie Spray]

so….I am looking at this differently. I thought she asked how wide a piece could she fit in a kiln with an 11" interior, correct? I am talking width here not height of mold.

So for depth …..with a top firing element with 9" depth the piece would be perhaps 7" from heat source unless it is a deep mold. I am picturing a mold that is no more than 2" deep……. 2" plus 1/2" stilt under shelf., 1" shelf is 3.5". 9" deep kiln would give you 5 1/2" clearance.

maybe I need more coffee…..

I agree with you in that there are so many differences involved but I would safely put an 11" mold in this kiln just having a slight clearance to the sides for shallow work which somehow is what I had envisioned them asking!

[loricrawford]

Thanks for you answers. I was asking how much clearance on sides and from lid if I were for instance going to make a small vase or bowls. I thought a couple of inches would be important. My kiln is side firing by the way. Thanks you so much for answering my question.
Lori

[Yardic Glassworks]

You also need to keep in mind the expansion that takes place in both the kiln bricks and the mold itself when it heats to top temp. An 11 inch mold that fits in an 11 inch kiln when cold will likely end up in several pieces after firing.

[Morganica]

so….I am looking at this differently. I thought she asked how wide a piece could she fit in a kiln with an 11" interior, correct? I am talking width here not height of mold.

So for depth …..with a top firing element with 9" depth the piece would be perhaps 7" from heat source unless it is a deep mold. I am picturing a mold that is no more than 2" deep……. 2" plus 1/2" stilt under shelf., 1" shelf is 3.5". 9" deep kiln would give you 5 1/2" clearance.

maybe I need more coffee…..

I agree with you in that there are so many differences involved but I would safely put an 11" mold in this kiln just having a slight clearance to the sides for shallow work which somehow is what I had envisioned them asking!

Probably not with a side-firing kiln, though--you'd be right up against the elements. And Tim's right--firebrick expands quite a bit, so there's a fair chance that you'd damage an 11-inch mold. In addition, you'd make it rather tough to get the mold/glass in and out of the kiln without damaging the soft firebrick (or your fingers). The less space you give for air circulation, too, the longer your mold/glass will take to reach thermal equilibrium. And that's especially critical in a side-firing kiln. I'd also worry about wearing out my molds prematurely, and good molds ain't cheap.

Rule of thumb: The mold should never have a footprint bigger than the shelf supplied by the kiln manufacturer. Ideally, it's at least an inch smaller. That doesn't mean you can't push the limits--we all do--but it shouldn't be a regular thing.