Kiln weirdness

While I was away at my last class, I did a slump firing of a bowl and plate into their final shapes.  When I got back and opened the kiln, I had a rather interesting surprise.  The two pieces pulled away from the edges of the mold to create sort of a ruffled edge to the pieces.  Very strange.  If you want to get a better look at the 2 photos, just click on the thumbnails and a larger image will open.

I'm not sure what has caused this problem, but I think that it looks good in the square plate and not so good in the heart bowl.  I'd love for you to leave comments on the pieces to let me know whether you like the effects or not.  Depending on the responses I get, I might give refiring the heart bowl a try so that I can get a better edge on it.

As I was washing the mold release off of the two pieces, an idea hit me that it might have been caused by the fact that I had not cleaned off the mold release from the molds in between firings.  After I had a firing disaster from not enough kiln wash on the shelf a while back (see this post for details) I thought that more release agent between the glass and the mold would be a good thing and haven't been cleaning off the shelf or the molds in between firings.

That buildup of kiln wash/mold release might have caused the glass to be less bendable around the edges where the release had stuck to the glass as it slumped and then cooled.  Since there isn't any release on the center of the glass pieces and there is some around the edges, it seems kind of logical.  But there I go, thinking again.  It usually gets me into trouble...  I think I'll go ask in the LinkedIn Glass Artists forum...  If there are any glass artists reading this post that have some ideas on the problem, please do leave me a comment!

Comments

[Randy Wardell]

Note: This is a copy of my post on the LinkedIn Fusing Forum.

Hi Pilisa:

First both of your designs have a sophisticated simplicity – well done! I especially love the effect of the patterned dicro on clear. The thing about fusing issues like this is there are often more questions than answers and when we do come up with a possibility it is nothing more than a best guess until it is tested. So with that caveat here are my questions and guesses.

1. Have you used these molds before with success? (I’m going to assume yes)
2. Were both of these pieces fired in the exact same kiln firing? (again, I’m going to assume yes)
3. Were the 1st fired flat blanks 2 layer plus design layer making them thicker than 1/4”? My thinking here is there may have been too much material (i.e. glass thickness) for the rather extreme angle/curve on the heart mold. Problem is that doesn’t explain the square plate.
4. So maybe it was not enough time at the slump soak temperature to allow gravity to do its thing.
5. I noticed that the heart blank was very close to the mold edge on the lower right side. Did the glass slump over the edge ever so slightly over there? This could cause a pull on the glass holding it to one side and have an effect on the slump. I would be curious to find out if you’ve ever fired a slightly smaller blank with success? (I would suggest 1/4” smaller than the mold all around).
6. Perhaps the type or color of glass you used created a variable. I have found that some glass seems to be more relaxed and will slump into the mold faster and more completely. I do not have any scientific explanation for this effect (I’m not a chemical engineer and I don’t play one on TV) it’s just something that I’ve noticed with my own firings.
7. As for your kiln wash build up theory. The glass has to be free & loose on the mold to allow it to slip and move as it sinks into the mold (see my #5 point). It is possible that the kiln wash somehow got stuck to the glass on the way up to slumping temperature and therefore the glass was stuck to the mold and could not fully slump. However kiln wash usually gets stuck to the glass at full fuse temperature and not at slumping temperature so I think this is unlikely the cause.

So my best guess is either the firing was not hot enough or long enough at the slump temperature. Or the glass was too thick to allow it to fully slump into the extreme curve. Actually it could be a combination of both of these. If the glass was thicker it would need a longer soak to slump.

Since you can’t use these pieces as they are anyway then I think it’s worth another shot to slump them using a slightly higher/longer slump program. And do it at a time when you can be there to open the kiln at slump temp to take a look. You may have some devit issues because it’s a second slump firing but at least you’ll find out if they will actually relax fully into the mold.

Let us know your results on the refire.

[Lanvin Flat Shoes icon]

You may have some devit issues because it’s a second slump firing but at least you’ll find out if they will actually relax fully into the mold.-Lanvin Flat Shoes icon