screen melt sticking to mold

[Thomas Decker]

I just did a screen melt, onto a piece of clear turned out great , turned out to be about 3/8 thick I cold worked it and took it up to a full fuse, ended up with a really pretty piece . problem is when I slumped it in a ceramic mold the edges caught in a couple of places and it did not slump evenly. I flattened it out and decided to re slump it in stainless steel and the same thing happened leaving some needle points and some slight distortion . both molds were properly kiln washed and the slumping temp was a ramp of 200 dph I have slumped many things before in the same molds and have never had this happen How can I avoid this in the future ? Thanks

[Brad Walker]

How high are you firing and how long a hold? What's the shape of the mold? What kind of kiln wash/boron nitride did you use on the mold?

[Thomas Decker]

for this one I went to 1240 and held for about 20 min I used primo primer on both molds . on the stainless I heated it up first and than sprayed several coats of the primer until I got a nice smooth coat I used 1240 because when I flattened out the first slump it did not flatten until 1230 also this was a pretty shallow slump With smooth sides.

[Kevin Midgley]

What is probably 'catching' you is the additional thermal mass and physical mass of the piece on those molds. Something thick will hold the temperature longer, and with the additional weight, effectively tear its way through the coating that was applied to the mold. Additionally if you are using screen melted opalescent glass that has been further devitrified than it normally is from being originally made at the glass factory, the loss of the glassy properties will make it want to grip and tear that mold coating. Where it moves the most will cause it to stick the most.

Modern commercially made kilns are way overpowered and enable to you to do work fast and furiously rather than low and slowly. You spend the time you "save" rapidly firing the glass trying to figure out how to clean up all the sticking points and repairing damaged molds.

As I was writing the above you posted a response.
Primo has clay in it which can cause sticking but the above message I wrote holds true. Why do you need to have it take a mold shape quickly?
Just trying to save time that you shouldn't try to save?

[Thomas Decker]

actually I thought 200dph was pretty slow. what temp would you suggest

[Kevin Midgley]

It isn't so much that the 200 per hour is fast but it is faster/more heat, than required. It is the total heat work that counts and the residual heat that continues to work on the mold beyond which that is required to have the glass form.
Your time and temperature formula is wrong for that combination of glass and mold.
I never suggest 'schedules' and think those that do, are doing a disservice for the kiln owner has to learn their own set of time and temperatures that work for their kilns and molds.

[Thomas Decker]

Keven after reading your reply, I re examined both the mold and the piece, and I noticed that the areas of the glass that are affected are indeed the thicker parts of the piece. and there are small pin point holes in the kiln wash in the mold. can this be avoided in the future with Boron nitrate and a slower processing speed?

[Kevin Midgley]

kiln wash applicator error.
kiln wash can be hand smoothed prior to firing which will break open those fragile air bubbles.
Bullseye type kiln wash tends to be less sticky as noted before on this board.
Once converted to Boron you can't go back without sandblasting the surface.
Boron can be brushed on as well if you get the liquid formulation.

However without my providing a schedule you have learned to observe carefully what has been going on and gained an understanding of 'kiln work' in your kiln.
Now you can try again with the warning that a high temperature pot melt is probably worth 2 of Bullseye glasses 3 guaranteed firing lives.

[Thomas Decker]

THANK YOU