surface problem on casting -

surface problem on casting

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debbie t
Posts: 9
Joined: Tue Mar 23, 2010 5:11 pm

surface problem on casting

Postby debbie t » Mon Jul 25, 2016 11:44 am

I have been experiencing some surface problems on my castings. has anyone had this problem before that can tell me why this is happening and how I can avoid it.
I use Bullseye frit with Ransome and Randloph casting material. Just switched to dust free bandust product.
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surface problems
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Thank you Debbie
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didn't quite melt properly
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Posts: 46
Joined: Mon May 21, 2012 10:34 am

Re: surface problem on casting

Postby Jenna » Wed Jul 27, 2016 11:48 am

Hi Debbie, that looks like devitrification (also referred to as devit). Search thru the archives and you will find plenty of posts about how and when it is formed and also how to prevent it from forming. Chances are you can address it by changing your firing cycle. Overspray (spray A etc) will not work in preventing devit in castings. Haven't tried it but apparently if any of the glass sprayed with the borax mixture is in contact with the mold, the glass will stick horribly.

What kind of kiln wash are you using? Do you prefire your molds? Are you using distilled water to make your molds?

I use the same R & R product as well as bullseye frit and have mostly eliminated devit from my castings. It took quite a bit of experimenting and some serious frustration along the way. You too will get there. When I do still get devit, it is usually on the upper face of the casting where I touched the frit with my finger, leaving behind some of the natural oil that is on our hands.
Have fun reading thru the older posts, there is a lot of good info there.
Post back with any further questions.

Kevin Midgley
Posts: 698
Joined: Mon Mar 10, 2003 11:36 am
Location: Tofino, British Columbia, Canada

Re: surface problem on casting

Postby Kevin Midgley » Wed Jul 27, 2016 1:00 pm

The answers are all here should you follow each of the threads through to their various ends.
Simply put use crystal for casting if you don't want problems. Then of course you have other problems in your studio when you start cold working such as lead dust.
Several hours of reading here.
If you feel the information here is worthwhile to you, both Brad and I would be more than happy to receive cash donations or pieces of your artwork :lol: PM me for my address! :lol:
Be sure to follow each and every link found.

Posts: 46
Joined: Mon May 21, 2012 10:34 am

Re: surface problem on casting

Postby Jenna » Wed Jul 27, 2016 6:23 pm

Hi Debbie, a few things to keep in mind and focus on while you're reading the archives...

How fast does your schedule go thru the devit temp range? 1270 deg F ( conservative low-end of devit range) to 1390 deg F or so (high range?) Since you are casting, the mold provides some insulation so these deg. ranges are relative to your kiln and the thickness of mold. What matters is when the GLASS sees the heat. keep this in mind when tweaking your schedule, there could be a 10-20 minute delay before the heat reaches your glass.
You want to heat and cool as fast as possible through this range to minimize devit.

Pre firing your molds in a vented kiln takes the burn-off of the casting material out of the equation. this could be one of the many sources of devit.
It took me a long time to come around to this, I will save you some time by highly recommending this pain in the ass step in the casting process. Worth the extra step.

Lastly, from looking at your pictures, I'm thinking you are not holding your top fusing temp long enough. It doesn't look like the frit has fully 'melted' enough to fill out the detail of your mold. Might want to increase your soak time at full fuse.

debbie t
Posts: 9
Joined: Tue Mar 23, 2010 5:11 pm

Re: surface problem on casting

Postby debbie t » Wed Jul 27, 2016 9:55 pm

I think I am not holding the melting temp long enough, did that and had success, I will also heat my molds to burn them out first. Thanks, Deb

Posts: 1079
Joined: Mon May 19, 2003 6:19 pm
Location: Portland, OR

Re: surface problem on casting

Postby Morganica » Mon Aug 08, 2016 12:03 am

Yes, you have devit on the glass, and I'm willing to bet that the problem starts before the glass ever goes into the kiln. What you think is partially melted glass is actually crystallized material from the glass (AKA devitrification). Devit isn't quite as cut and dried as most people think--it's usually caused by a combination of things, but the biggest in casting is chemical contamination from the mold. The "devit zone" is simply the place that soda-lime crystal growth happens fastest, but with the right conditions, devit can grow at virtually any temp.

Lengthening the time at casting temp will make your devit worse, not better. Devitrified glass is no longer glass, it's crystals, and those crystals need a lot more than 1500-1600F to remelt (and if they do, they'll be a different, possibly incompatible, glass). Crystals need energy to grow, so the more you heat that glass, the more crystals you'll get. The only way to stop it is to prevent it from happening:

--Follow refractory mix directions exactly. R&R has some very precise instructions on measuring, mixing, and building their molds; the resulting mix can seem too thick and dry, but trust it. Wetter, runnier mold mixes are weaker, and more likely to scum up in steam-out or break off and mix with your glass...all of that can exacerbate devit. Go by weight, not volume, when measuring all ingredients. And make sure you're mixing long enough, with a mechanical mixer (I use paddles on a hand drill I keep just for that purpose).

--Make sure the mold is absolutely fully cured. Refractory plasters will seem ready for the kiln in a few hours, but the mold hasn't reached its full strength and hardness. I cover mine with loose plastic wrap (very loose), and let them sit for 24-48 hours before I start the dying process. Plastic wrap retards loss of surface moisture while curing takes place, lessening the likelihood of cracks (which can release plaster into your glass and cause devit). If you've got instructions from R&R, follow them and err on the side of caution. (You can call Ralph over at R&R and ask for advice; he's very knowledgeable on this stuff).

--Watch how you steam out wax--it's very easy to oversteam, especially if your mold isn't fully cured. If you can run your finger along the inside of the steamed-out mold and have it come away looking milky, you've probably steamed too much. That milky can mix with/coat the surface of glass particles at the surface, which will scum up the glass and usually cause devit. (One way to avoid oversteaming: Use hollow wax molds and ultra-flexible soft silicone forms (patterns). Big, solid chunks of wax take longer to steam and increase the chance that you'll damage the plaster, and silicone patterns eliminate steaming altogether).

--Let the molds rest after steaming, before you apply any dryout heat.

--The best results usually come when you completely remove both mechanically and chemically held water from the mold. Unfortunately, that also means taking the mold up to 1100F or so and cooling it back down, which makes the mold dangerously fragile (R&R 910/965 are actually pretty good for this). The better compromise is usually getting out the mechanically held water (kiln at more than 300-350F with the lid open an inch or more and all vents open/fans on), cooling and loading your glass, and then keeping the kiln vented until around 1150F.

--Do NOT mess with the surface of a cured mold, especially if you're using something like R&R910. Those plasters form a lovely skin on the surface of the mold that prevents the strong-but-sandier subsurface from interfering with the glass. If you damage or wear away that surface, you're much more likely to introduce plaster particles or even sand into the glass...which also causes devit. It's tempting to come back with a little more plaster to patch loose spots or bubbles in your mold, but the longer it's been since the mold has set, the more chance that the stuff will loosen and cause devit.

--Devit forms on the surface, so the more surface area of glass concentrated into a small area, the greater the chance of devit. What this means is that you usually have to work at devitting smooth castings made with billet, but intricate pate de verre pieces made with powder can devit if you look at them wrong. (Glass powder is pretty much nothing but surface area). I can't use R&R910 with pate de verre (although others do)--apparently my application method scrubs up the surface too much and I wind up with pieces that look like they've been in a sandstorm. I have more success if I mix up a very thin kilnwash, dump it into the cured-but-not-dried mold and let it sit for about 5 seconds, then dump it out. That provides enough kilnwash for good surface protection/release, but still allows fine detail.

--Extremely sharp points (i.e., narrow holes) in the bottom of a mold are like plaster magnets; any little crumb of mold plaster is going to wind up down those holes and be devilishly hard to remove. Those areas are already prone to devit (high surface area ratio), so don't make it worse: Cover those areas (I use a little piece of paper towel) when you're working. It helps to have a little electronics vacuum and handlight to make sure you've removed all the bits.

Try some of those suggestions; let me know if they help.
Cynthia Morgan
Marketeer, Webbist, Glassist

"I wrote, therefore I was." (me)

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