Exploding during Glaze Fire - using glass

[Oddoneout]

I hand made a set of coasters, in a white clay body that I have been using for the last two years. They were about 3.5 inches round, 3/16th thick and bisqued to 04. I glazed the rims with Amaco glazes that I have not had issues with. I used a clear glaze on the center portion (it was at SG 1.2) and I had waxed the bases. I put them in my kiln and put blue glass chunks on the center. I fired to Cone 6. It was pretty fast about 8 hours (I have  a kiln sitter, so I am basing my time off the timer and when I was upping the heat). I followed my basic cone 6 firing schedule that has always worked. I let the kiln completely cool before opening. When I did all most all of the rims on the coaster has broken off and left the area. I found a piece stuck in the kiln brick and on all 3 shelves and the bottom (this is fun because now I have to figure out how to get it off without damaging the floor. I can only think that since I had wax on the bottom and glaze everywhere else that there must have been water that was not burned off when the glass melted and this caused the rims to blow off.

If anyone has any advice, I really like the look and would like to me a set again (they were supposed to be a Christmas Gift, and I still want to give it, although not for a little while).

Thank you,

Z

[Kelly in AK]

Could you post a picture of the broken pieces?

 I had some students try this. The first pieces were small, coaster size, and very shallow. They came out great. Next load had larger pieces, saucer sized, some with deeper rims and a thicker layer of glass. Most of those were in pieces when I unloaded the kiln. Broke to bits on cooling. 

[neilestrick]

The glass has a  very different COE (coefficient of expansion- different shrinkage) than the clay and glaze, and it most likely put a ton of stress on the coasters and blew them apart.

To get the piece off the bottom, try sliding a pallet knife/scraper under the chunk. You may lose a little bit of the floor but that won't hurt anything. If it has melted into the floor a bit, put a thick layer of kiln wash on it for the next few firings until it stops melting.

[Min]

Firing glass on clay has come up a few times on the forum. General consensus of forum members is this isn't a good idea. Several threads on this topic, link below to one of them.

6 hours ago, Oddoneout said:

I can only think that since I had wax on the bottom and glaze everywhere else that there must have been water that was not burned off when the glass melted and this caused the rims to blow off.

Don't think this would have been why. Reason why the melted glass has craze / crackle patterning is because the glass expands and shrinks at a vastly different rate than a claybody. COE or CTE (coefficient of thermal expansion), same thing, different name.  Sounds like with your original coasters the glass was much thinner than the ones that shattered in your latest firing. This doesn't mean it's safe practice to just put the glass on more thinly, still subject to stresses and the risk of slivers of glass dislodging. 

 

[Callie Beller Diesel]

Know that despite how the tone of this might come across, I do sympathize with your ruined project. I mean none of this as shaming, only as information.

You have a relatively wide piece compared to the thickness, no foot rim, a very thin cross section and you glazed only the inside and the top of the rim. You don’t state it, but I’m  going to guess that the glaze layer was a comparable thickness to the cross section of the clay. That alone can be a solid recipe for dunting, even if you haven’t had previous problems with that glaze. That scenario can actually a way to set up a stress test for clay glaze compatibility. 

Add to that chunks of glass of unknown providence, which,  as mentioned above has a VERY different COE than either your glaze or your clay, and fired it far past the maturation point of the glass. I’m not at all surprised these coasters pulled themselves apart with some authority. 

It is, and always will be my opinion that it is ill advised to combine chunks of “found”  glass and ceramic material, ESPECIALLY in functional items. I have stronger words for people who teach this as a beginner project. I know lots of people do it, but it’s still a bad idea. You have unfortunately found a perfect storm example of why.

I do concede in that other thread that there are some circumstances where combining clay and glass do work, or where cullet is included as a glaze ingredient. But to make it work properly requires a very strong technical understanding of the differences between clay, glaze and glass. I will forever disagree strongly with anyone who just melts glass into the bottom of a clay dish. 

If you want some nice coasters to be able to gift for Christmas, even the most technically broken glaze would be a better option than glass. This would be a lovely opportunity to go on glazy, and test out some of those fun, runny zinc glazes that make crystals. I saw today that Birdie Boone and Osa Atoe are working together on just such an example. You could get some really fun effects with none of the headaches of using glass. If you’ve never mixed a glaze before, this is a very simple recipe, and would be a good place to start. Some of the ingredients are a little more expensive, but you probably don’t need large quantities, so it’s still pretty affordable. 

 

 

[Roberta12]

4 hours ago, Callie Beller Diesel said:

Know that despite how the tone of this might come across, I do sympathize with your ruined project. I mean none of this as shaming, only as information.

You have a relatively wide piece compared to the thickness, no foot rim, a very thin cross section and you glazed only the inside and the top of the rim. You don’t state it, but I’m  going to guess that the glaze layer was a comparable thickness to the cross section of the clay. That alone can be a solid recipe for dunting, even if you haven’t had previous problems with that glaze. That scenario can actually a way to set up a stress test for clay glaze compatibility. 

16 hours ago, neilestrick said:

The glass has a  very different COE (coefficient of expansion- different shrinkage) than the clay and glaze, and it most likely put a ton of stress on the coasters and blew them apart.

What they are telling you is true.  I used to melt a lot of glass into the bottom of small pots.  I actually had a small sliver come loose and cut a person.  Not badly, but that stopped me in my tracks.  And @Callie Beller Diesel and @neilestrick are correct about the explosion.  I had a smaller incident with glass in the kiln.  It's simply not a safe practice.  If you have seen commercial coasters with some sort of melted glass in them, I surmise that they were treated with a form of resin to seal the glass.  I have one on my desk.  Which is what prompted me to try it in the first place!  Live and learn.  

Roberta

[Chilly]

My way of wording the same info that @Callie Beller Diesel and @Roberta12 have said is:

We have enough problems getting clay and glaze that are sold as compatible, to be compatible, without adding an ingredient that is scientifically not compatible.

You wouldn't dream of putting diesel fuel into a petrol-engined car (or vice-versa), so why would you want to mix clay and glass. 

Petrol and diesel both come from crude oil, but are not compatible.  Glaze and glass may both contain silica, but they are also incompatible.

[Jeff Longtin]

The pottery shop most associated with this process is Dock 6. Kerry Brooks is the potter and she applied this process to coasters as well. She spent a year, or two, developing the process and the glazes that go with it. 

Again, she spent a year, OR TWO, developing this and I know she had a lot of failures. (Her studio/shop is a few miles from mine. I know she experimented with glazes, with firing temperatures, and with types of glass.)

Once she found success technically she found success in the marketplace. (Anthroplogie ordered 1000's.) 

Having seen failed experiments with coke bottles, in high school and college, I have never felt inclined to attempt anything like this. She found the sweet spot and her coasters really are a thing of beauty.

[Callie Beller Diesel]

@Jeff Longtin I think I have a fridge magnet of hers. My hubs went on a business trip to Chicago, and brought back a magnet in the shape of Illinois because it was pottery related. (Awww!)

It looks to me like she’s also put some epoxy or clear coat of some kind over the melted glass on this piece in particular. 

[LinR]

My understanding is that if glass on clay does not fail in the kiln it will fail eventually.  I have tried it a few times on non functional pieces and had loads of failures.  Lin