WarmGlass.com

Hi all. This is my first post here and have zero experience with the art of warm glass. Like most other newbies, I've read and absorbed dozens of hours of reading, but have come back full circle to some of my original questions.

I am interested in warm glass mostly to cast one particular item with minor shape variations, so at this stage I am just trying to make sure I am on the right path. Basically, here is what I am trying to accomplish:

I need to create the solid glass equivalent of a plastic, old-fashioned shaving brush handle. If you can't picture one, it's cylindrical, about 1" wide, and 2" tall (basically it's shaped like a pillar that supports a building). Most everything I've read references many techniques using store-bought molds, but I can find no molds even close to this shape, and I am expecting to need about a dozen slight variations of this shape.

Therefore, I need to know the most appropriate molding material and process that would be best suited to create a mold to support this shape. The texture of the final product needs to be extremely smooth since it's a product that will be handled often. I am picturing a two part mold that I can fill with COE 90 frit to fill the cavity, and fire in a small Evenheat Pro STP kiln. This type of project seems to be a little off the beaten path, so I'm having some difficulty finding resources that describe my requirements.

At this point, I just need to know that I can make a mold reasonably easily for this oddball shape, and what material / process I should use to create a mold that can survive at least a few firings. Obviously I am going to start off with something much more conventional and basic before I attempt this to gain experience first, but I just want to make sure I have the right approach in mind for my end goal.

Thanks,

Sean

I'm not sure why you need a two-part REFRACTORY mold but if you're making multiples you may need a two-part MASTER mold. The refractory mold is the one that contains the glass and goes into the kiln. The master mold makes the wax (or whatever material) copies of the original. You can start with the basic handle shape and embellish with whatever variations you want.

So...first, I'd strongly recommend that you buy Kervin & Fenton's "Pate de verre & kilncasting of glass" book. It'll give you a great deal of good information on casting. You also want to review the Bullseye technotes (available free on their website) around lost wax casting, working with thicker glass, and similar topics. (and you can check out my blog for tips on casting and supplies)

Second, mold engineering depends on the shape and detail of the model (in this case a brush handle), how you plan to deliver the glass and how transparent you want it to be. This shape is symmetrical, so a single-part mold is probably sufficient. If you use plaster/silica or something similar to construct your refractory mold (which I'd recommend), it should crumble away after firing, so you'll have no problem getting the glass out. You'll make your "patterns" (the copies of the original model), pour a plaster/silica mold around them, remove the pattern, fill the mold cavity with glass, and fire.

Alternately, you could construct a permanent refractory mold out of ceramic or a casting cement. Problem is, it would be more difficult to build and use (you need a high-temp release on the mold or it will stick to the glass), it must be two-part and you'll be doing extra coldworking to smooth the seams. Such a mold would also be more expensive, and make it harder for you to make variations on the original (permanent refractories are really geared to making identical copies of the original).

Either way, your mold must deliver enough glass, especially if you're casting with frit. Frit can compact to less than half its original volume during firing, so if you simply put glass in the mold cavity you'll wind up with half a shaving brush. I've got a blogpost that explains this in more depth: http://www.morganica.com/bloggery/2012/ ... it-volume/

To compensate, you do one of three things:
--Build a reservoir into the mold to hold the extra glass
--Design the mold so you can add more glass once the glass has melted down (a process called "charging" the mold, and it involves opening the kiln at top temperature and dumping in more frit)
--Devise a reservoir system to drip the additional glass into mold until it's full

What that means is that your mold is probably going to be taller than you envision. If your brush is two inches tall, the basic mold will probably be about three inches tall. It needs at least a half-inch of clearance from the kiln floor to allow air to circulate properly, and the glass reservoir system needs another 3-5 inches. Add another inch or more in clearance from the kiln ceiling, and that means your mold assembly will need 10 inches or so.

Transparency will also figure in. Frit traps air between particles as it softens, creating lots of bubbles. The smaller the frit particles, the smaller (and more numerous) the bubbles. Big bubbles will show up as, well, bubbles. Smaller frit bubbles will trap light and make the glass look translucent or opaque--it will look more like alabaster than glass. Dripping the glass into the mold from a reservoir is the most likely to produce a bubble-free, transparent piece of glass.

It's also the best way to minimize coldworking, i.e., the grinding and polishing of a casting after it's been demolded and cleaned. Soda-lime frit (such as COE 90 glass) has a tendency to produce a cloudy or scummy surface when placed directly in a plaster/silica mold, and that needs to be ground off. Billet (solid glass chunks) tends to produce the cleanest surface. In any case, you'll also need to coldwork off any extra bits of glass and rough parts of the casting.

Thank you so much for the reply, Cynthia. I will definitely look into the book you recommended. I also really appreciate you pointing out the the need for charging the mold if I am using frit due to shrinkage and the resulting bubbles. Most importantly, the need for a potential 10" working area inside the kiln. The kiln I was considering would have been way too short for this.

Thanks a bunch!

Sean