Questions about casting frit

[Amanda Walker]

Hello all! New here and very excited to read through the forum and learn.

I've been kiln-casting with billets with good success for a few months. I've had problems with frit, though, and I'm pretty green at it. I've been making glass boxes with glass tops, and putting small 3D sculptures on the box tops. I usually cast the top itself from billet glass, and try packing dry frit into the small 3D sculpture on top of the lid. About 50% of the time, I get a very sugary effect from the frit that distorts at least part of the small sculpture. The rest of the time, it's fine. The molds are of comparable size, the firing is schedule is the same, the frit is compatible with the the billet glass (all Bullseye, with care to look at the reaction chart). The billet glass always casts beautifully- never a problem.

And I've been using Ransom and Randolph 965 investment, which I love. No cracks, easy to break off, no reinforcement needed, about $1 per pound.

I have a 6" X 6" X 5" R & R mold of a lily (lost wax). The water weight of the displaced wax piece was about 200 g. I'd like to pack it with frit (white at the tips of the petals, pink at the base, yellow or orange in the middle). I did make a reservoir at the base of the flower (top of the mold) so I could put in extra frit to allow for all the air space.

Question 1: Is dry packing frit causing my "sugar effect" problems? I would really like to see this be a smooth, non-sugary, opaque piece, but I don't think you can fire the R & R wet.
Question 2: What is the best firing schedule for such a mold? Most of the advice/firing schedules I've seen have been for small, jewelry-sized frit molds, and this is pretty hefty- the mold weighs about 10 lbs (because the piece has curving petals, so it takes up a lot of space, though not a lot of volume).

I'd love any advice from frit experts before I spend the time and money on this piece!

[Morganica]

Hello all! New here and very excited to read through the forum and learn.

I've been kiln-casting with billets with good success for a few months. I've had problems with frit, though, and I'm pretty green at it. I've been making glass boxes with glass tops, and putting small 3D sculptures on the box tops. I usually cast the top itself from billet glass, and try packing dry frit into the small 3D sculpture on top of the lid. About 50% of the time, I get a very sugary effect from the frit that distorts at least part of the small sculpture. The rest of the time, it's fine. The molds are of comparable size, the firing is schedule is the same, the frit is compatible with the the billet glass (all Bullseye, with care to look at the reaction chart). The billet glass always casts beautifully- never a problem.

And I've been using Ransom and Randolph 965 investment, which I love. No cracks, easy to break off, no reinforcement needed, about $1 per pound.

I have a 6" X 6" X 5" R & R mold of a lily (lost wax). The water weight of the displaced wax piece was about 200 g. I'd like to pack it with frit (white at the tips of the petals, pink at the base, yellow or orange in the middle). I did make a reservoir at the base of the flower (top of the mold) so I could put in extra frit to allow for all the air space.

Question 1: Is dry packing frit causing my "sugar effect" problems? I would really like to see this be a smooth, non-sugary, opaque piece, but I don't think you can fire the R & R wet.
Question 2: What is the best firing schedule for such a mold? Most of the advice/firing schedules I've seen have been for small, jewelry-sized frit molds, and this is pretty hefty- the mold weighs about 10 lbs (because the piece has curving petals, so it takes up a lot of space, though not a lot of volume).

I'd love any advice from frit experts before I spend the time and money on this piece!

Probably not enough info (what was your schedule, for example?), but I'll give it a shot. The goal with most glass casting molds is to get the best possible detail and surface with the least adhesion (i.e., sticking to the glass after firing), and I've found that the best results usually come from billet dripped into the mold. The worst is always wet-pack powder against the mold surface--other techniques are somewhere in between. If I combine techniques, as here, I see clear evidence of that:

This is wet and dry pack frit for the fingers and face, billet casting for the background--the mold lifted off the billet perfectly and gave me the detail I wanted. The PdV areas tended to stick and had to be power-washed away. That's just the nature of the beast--fritcasts expose a lot more glass surface area to the mold, and if you're packing the powder against the mold surface you're both abrading and integrating the grains with any plaster coming off the mold.

I found that the R&Rs, especially 910--965 not quite as much--didn't like the abrasion of wet powder against mold surface. Once its surface was broken, little grains of mold sand would embed into the surface of the glass when I used frit. I'm not talking about a grainy appearance to the glass, but rather particles of sand barely stuck to the surface, enough that they needed to be pried or coldworked off. I found I could prevent it by sloshing any good kilnwash into the mold then quickly pouring it back out. That prevented the sand issue while keeping detail.

I don't think that's the problem here--couple of possibilities. First, if you're using larger frit sizes (fine to coarse) against the mold surface, grainy distortions are far (FAR) more likely. I always use a layer of powder at the surface, even if 1-2mm in I'll be using larger frit, and it preserves the surface and detail. Here's a comparison using identical waxes with different form factors of glass (sorry for the lousy image). As I said, the powder pieces have a more matte finish and you can still see plaster stuck to the surface. The billet pieces don't do that.

The second possibility is that you've got devit. Frit casts are far more prone to devit simply because there's more surface area from all those particles, and any grains of plaster that embed in the surface can act as nucleation sites. Extreme devit looks a lot like piles of sugar crystals. This is a badly devitted piece from the same molds as above:

I've found that wet pack is a bit more likely to produce devit than drypack, possibly because the mold's a little soggier. (If you can touch the mold with your finger and have it come away wearing a milky coating, you've got problems)

If I see devit I (a) look at my schedule and potentially reduce my processing temperature, (b) am very careful not to abuse the mold by oversteaming or abrading it, (c) ensure I've allowed at least 24 hours for the plaster to cure before I start a steamout (or if I'm using a direct clay model, that I'm using wet clay that can be easily removed), and (d) examine my mold-making process and make sure that my mix isn't too wet--wet molds are weaker to begin with, and I've found that with steamout, they're FAR more likely to devit.