casting a stepping stone

[blaich]

Hi

I've been asked about the possibility of making a thick, round glass stepping stone. This is new for me and I wondered, is there a preferred thickness for stepping stones? I'm assuming that making a piece above a certain thickness is unnecessary but if the piece needs to be thick, I'm assuming that casting is the way to go, rather than fusing or pot-melting?

I've been doing fusing a while, but have never done any casting, so I'm wondering if anyone has suggestions, or could point me in the right direction to find out things like optimal thickness, firing schedules etc. It probably wont be a large piece, about 6-8 inches diameter. Aside from needing to be weight bearing, we are in a climate with fairly cold winters, so I'm guessing this is going to need a very long anneal. I'm thinking that I would probably need to use a steel mold and line it with fibre paper? What kinds of things should I consider? In case it is of any relevance, in terms of finishing the piece I don't have any large lap grinders or sandblasting equipment.

Thanks for any help.

[Morganica]

Bullseye published a very nice technote on glass casting and also has an annealing for thick slabs handout--both are must-reading if you're getting into casting, available free on their website. Probably the best US book ever written on the subject is Kervin & Fenton's "Pate de Verre and Kiln Casting of Glass," available on Amazon; I strongly suggest getting it if you think you might take up glass casting.

That said, if you just want to do a round stepping stone with no detail (a smooth cylinder), you can probably find a ready-made form to cast into instead of getting into the whole moldmaking thing.

Ideally, your form should have a negative draft, i.e., the opening at the top of the mold should be a bit larger than at the bottom, so that the glass slides out easily. If the sides are straight up and down you'll have to use an extra layer of fiber paper on the sides to ensure you can get the glass out.

The pure glass stepping stones I've seen are between 1.5 and 3 inches thick, with most around 2 inches. They'll need something like 60-70 hours in the kiln. Use a regular fusing schedule for the most part, but slightly hotter and quite a bit longer at the top process temp (the technote will explain that).

You'll also want to be careful of the colors you use--hot transparent colors like red, orange, yellow will go opaque with prolonged heatwork. And if you're looking for perfectly transparent glass, the bigger the glass chunk you start with, the fewer bubbles and clouding you'll get, as a rule.

Weather-wise, I've been told by the architectural guys at Bullseye that the dangers of extreme temps are primarily around water getting into crevices in the glass, freezing and then expanding to crack the glass. A perfectly smooth cylinder shouldn't have that problem. You could run into problems in summer if the stepping stones are sitting in the sun and the sprinklers come on, stuff like that. You want to keep the temperature on all parts of the glass within 50 degrees (or less).

[Mike Jordan]

If they don't care what glass it's made out of, you could use a sheet of tempered glass, break it into chunks and then fuse the chunks together inside of a stainless steel ring or even a sand mold if you want an odd shape. I've made several 3/4" thick by 14" across disks using a bunch of 3/8th thick tempered glass chunks from sheets I got very cheaply (I've seen it for free too). At some point I plan on using a simple sand mold to let it flow out to make bird baths out of. But you can do a lot of things cheaply with tempered or even non-tempered float glass.

Mike

[blaich]

Thanks both of you for your help! I appreciate it.

I think for this case, I'd just use a ready-made s.steel mold that I have which is straight up and down, so I'd line both the mold and the kiln shelf with fibre paper. Using a ready-made no doubt makes for an easier introduction into the casting world too.

I was surprised that the stepping stones are made so thick - that's a fair amount of glass in 2 inches, so thanks for that information. I'd imagined that only and inch would be required - about the same depth as a flagstone.

Re the bubbles...for an outdoor piece, am I right in thinking that you'd want to minimise bubbles so as to reduce the risk of cracking (air in bubbles heating at different rate to glass etc)? Or if the piece is really thick, would the air in the bubbles be insulated to some degree, so long as they aren't close to the surface?

I like Mike's idea of tempered glass, but in this case I'd have to use specific colours etc.

If anyone wouldn't mind answering a question from a business perspective....the pricing of thick cast projects seems difficult because it's not simply that you have the elevated power costs of the long firing schedule, but the kiln is tied up for several days, with no other saleable pieces being able to be fired in that kiln. Then, if the piece doesn't turn out right, you've got to fire another tying the kiln up for another 60-70+ hours. Do you simply price up the glass, time and power etc, and possibly allow for doing the job twice (hopefully no more); or do you also factor in lost production time because of the length of the time the kiln is tied up? What seems fair? Obviously there are other cost factors involved too, and it depends on how much you value doing the project itself, but I'm speaking roughly here.

Again, thanks for your answers.

[blaich]

The Bullseye schedule for casting on Tip Sheet 8 have long holds at 200F and 1250F, it says, to remove physical and chemical water. Is this referring to water from the casting mold or from the washed glass? I'm not sure I understand why such a long hold would be needed to remove water from glass. When I do pot melts I never hold at these temperatures and so far haven't had any problems (though my glass is usually dry). If using a pre-made mold and dry glass, is it necessary to have these long holds?

Thanks.

[DonMcClennen]

An alternative to casting is outlined by Bullseye tech sheet "Working Deep"....you wind up with a stack of compatible glass (10 to 14 layers) fused with minimal bubbles, excellent clarity, and the oportunity to add internal design layers...(see Dorthey Hefner).........final firing is about 72hrs....I've done several....however I can't imagine someone paying for one of these then stepping on it!
Don

[Morganica]

The Bullseye schedule for casting on Tip Sheet 8 have long holds at 200F and 1250F, it says, to remove physical and chemical water. Is this referring to water from the casting mold or from the washed glass? I'm not sure I understand why such a long hold would be needed to remove water from glass. When I do pot melts I never hold at these temperatures and so far haven't had any problems (though my glass is usually dry). If using a pre-made mold and dry glass, is it necessary to have these long holds?

Thanks.

It's to remove water from a plaster refractory mold, not the glass. As Don says, simply stacking the glass in a premade mold (or damming it) and firing works too--it depends on whether you need a surface texture/design, what shape you want in the final piece, and how much coldworking you plan to do.

[blaich]

Yeah that's what I'm thinking....72hrs of kiln time (assuming it works first time) amounts to a very expensive stepping stone. I'm wondering if a regular thick fused glass circle shape would work sufficiently well as a stepping stone, provided that it had an even concrete substrate underneath? It wouldn't be as strong as a 2inch cast piece but if it's more an aesthetic piece (which I think it would be) rather than part of a pathway, it might be ok.

Regardless of the outcome business-wise, I think I am now sufficiently motivated to have a go and cast something for myself as a personal project, just to see if I can do it properly!


I want to minimize coldworking as I don't have any coldworking equipment other than hand diamond pads and a small regular stained glass grinder.

Thank you

[Kevin Midgley]

In the older parts of cities it is possible to sometimes walk on sidewalks that have thick glass tiles inset into them. These were inset to allow light into the building areas below the sidewalk. In a rainy city the one thing I remember about them was their being slippery to walk on.

[Morganica]

Yeah that's what I'm thinking....72hrs of kiln time (assuming it works first time) amounts to a very expensive stepping stone. I'm wondering if a regular thick fused glass circle shape would work sufficiently well as a stepping stone, provided that it had an even concrete substrate underneath? It wouldn't be as strong as a 2inch cast piece but if it's more an aesthetic piece (which I think it would be) rather than part of a pathway, it might be ok.

Regardless of the outcome business-wise, I think I am now sufficiently motivated to have a go and cast something for myself as a personal project, just to see if I can do it properly!


I want to minimize coldworking as I don't have any coldworking equipment other than hand diamond pads and a small regular stained glass grinder.

Thank you

If it is attached to a concrete substrate with some kind of adhesive (or just set into wet concrete mix), so that it becomes part of the concrete, the thickness really doesn't matter. At that point, it's a tile and you can make it a single layer if you like. If the glass is loose, so that the glass and concrete can move independently and water/other stuff can potentially get between them...then the glass needs to be stronger, i.e., much thicker.

[Mark Hall]

Stepping stones I've made using discarded blown objects work well. Every hot shop throws away glass that's perfectly able to be piled up inside a high-walled perimeter (dam) which can be any shape because it's made spur of the moment. Just make sure no rogue glass is added to the pile, and fire away! Unfortunately I've many beautiful stepping stones in our garden made from my many mistakes learning how to blow glass! I make them only around 1" thick, and they hold up well in this low traffic location. Better than throwing it away!