At My Wits End

[Randy W]

I love making pot melts, but every once and a while one would break on me or I would get some crazing in the surface. If I tried to re-fuse it , it would usually crack in half before I got to 500°.
I used to fire in an old ceramic kiln with a manual controller. (A knob that went from 1 thru 10) I had a hard time annealing with that kiln and figured that's where my breaking was coming from. One day I made an especially beautiful piece, the next morning I came into my shop to find it broken in half and full of small cracks. In a fit of childish rage I smashed it on the floor, called Ed Hoy's and ordered a new Evenheat coffen kiln with a Rampmaster2 digital hand held controller. Made some pot melts in my new kiln and I'm STILL having the same problem. I took a pot melt out of the kiln last Thursday morning, let it set for a couple of days and there were no cracks. Sunday morning I sandblasted the kiln wash off and everything still seemed OK. Later that evening it developed all kinds of cracks.

http://f1.pg.photos.yahoo.com/ph/74dodg ... kenPotMelt

The ones that crack in half I figure is an annealing problem, the ones
that get all those little cracks, well, I have no idea what's causing them. And why didn't they develop until after I sandblasted?
I'm using all Spectrum compatible glass. Here is my firing schedule for the pot melt.
AFAP to 1675°
Hold for two hours.
600° per hour to 1050°
Hold for 1HR 30 Minutes.
100° per hour from 1050° to 700°
Let the kiln cool on it's own, no venting.

For some reason this peticular color combination is giving me the most trouble. White, two shades of amber, clear and a little black. The cracking quite often is where there is white and clear together but not always.
Does anyone have any suggestions why this is happening?
Could it be the high temps, maybe Spectrum glass doesn't like to be that hot ?
Is it eh clear?
The sandblasting?
Annealing?

Naturally I have a gallery that's interested in them and wants a dozen pieces by the first of the year.
I'm at my wits end !

Thanks for the help,
Randy

[Brock]

Why the hold at 1050? Try holding at 960, then slowly ramp down. Brock

[jim simmons]

Why the hold at 1050? Try holding at 960, then slowly ramp down. Brock

Also, why not "afap" down to 960?
jim

[pclark]

Randy,
I am going to take a shot in the dark but, my guess is you hit the nail on the head by stating the cracks tend to happen between the white and another color. I know that white and black can cause problems when put together because black is such a soft glass and white is harder. Not to mention even though they are the same COE the colors absorb and release radiation. That being said I wish I could tell you how to fix the problem. My guess would be a slower cooling phase. Best of luck.

[DonMcClennen]

I was told by a glass blower (when I experienced the exact same problem you describe) that some glass( especially yellow) can change the coeficienct of expansion at high temperatures. Noting that the temperatures we use on pot melts is higher than we normally fire at, I accepted this theory and gave up trying. The pieces I had trouble with also had a mixture of white, clear, blue, and a little yellow. (all "96")
If you discover some other reason I would be all ears.
Don

[Lisa Allen]

Randy, I had some similar problems with my pattern bar pieces mixing transparents and opaques. For Bullseye, the specific annealing point for transparents is 990 and for opaques it is 935. To accomodat both, I started holding at 1000 for an hour to try to get the temp of the glass regulated before I started my descent thru the annealing points. Then I go 40dph to 960 (the average of the 2 annealing points) and hold for 2 hours, then 40dph to 700, then off. My pieces are twice as thick as yours, so that is probably too conservative for you, but it solved my cracking problems.

I would suggest finding out the specific annealing points of Spectrum trans and opaque and make sure that you accomodate both in pieces that contain both, by either holding at the average of the two or by slowing your ramp down much slower than 100dph thru the range that covers both.

Hope this helps. I know how frustrating the cracking can be.

Lisa

[Randy W]

I went down slow from 1675° to 1050° because I didn't know how fast my kiln cooled off and it really wouldn't hurt anything going slow.

I held at 1050° for an hour and a half because I wanted to make sure the temperature had equalized.

And I started annealing at 1050° cause I was just being safe. Maybe I'll give it one more try with a painfull slow anneal.

Last summer I did a pot melt into a 4" X 6" X 1" block using Spectrum white, clear and blue opal. When I cut the block the blue and white were OK but the clear had cracks about every 1/8".
I also did a block with Cathedral red, orange, yellow, a little black and clear and had NO problems. The red, orange, and yellow turned opaque but I had no cracks.

I like to add clear because it adds a lot of depth to the piece. I'll do a little research and give it another try using just opals and see what happens.

Thanks for all your help.
Randy

[Brian and Jenny Blanthorn]

I love making pot melts, but every once and a while one would break on me or I would get some crazing in the surface. If I tried to re-fuse it , it would usually crack in half before I got to 500°.
I used to fire in an old ceramic kiln with a manual controller. (A knob that went from 1 thru 10) I had a hard time annealing with that kiln and figured that's where my breaking was coming from. One day I made an especially beautiful piece, the next morning I came into my shop to find it broken in half and full of small cracks. In a fit of childish rage I smashed it on the floor, called Ed Hoy's and ordered a new Evenheat coffen kiln with a Rampmaster2 digital hand held controller. Made some pot melts in my new kiln and I'm STILL having the same problem. I took a pot melt out of the kiln last Thursday morning, let it set for a couple of days and there were no cracks. Sunday morning I sandblasted the kiln wash off and everything still seemed OK. Later that evening it developed all kinds of cracks.

http://f1.pg.photos.yahoo.com/ph/74dodg ... kenPotMelt

The ones that crack in half I figure is an annealing problem, the ones
that get all those little cracks, well, I have no idea what's causing them. And why didn't they develop until after I sandblasted?
I'm using all Spectrum compatible glass. Here is my firing schedule for the pot melt.
AFAP to 1675°
Hold for two hours.
600° per hour to 1050°
Hold for 1HR 30 Minutes.
100° per hour from 1050° to 700°
Let the kiln cool on it's own, no venting.

For some reason this peticular color combination is giving me the most trouble. White, two shades of amber, clear and a little black. The cracking quite often is where there is white and clear together but not always.
Does anyone have any suggestions why this is happening?
Could it be the high temps, maybe Spectrum glass doesn't like to be that hot ?
Is it eh clear?
The sandblasting?
Annealing?

Naturally I have a gallery that's interested in them and wants a dozen pieces by the first of the year.
I'm at my wits end !

Thanks for the help,
Randy

Have U done compatability tests on all glass ????????????????

Try n keep temps lower in the melt part creap up 2 temp rather than go fast n over heat

And soak a little as U go up

Have larger hole in pot

One thing appears 2 happen with this sorta thing is the glass can change due 2 the heat / pooring

U got a lot of complex stuff in some of those colours n they will wanna B buring out or just doing weird stuff

U need less time in contact with air it must get super heated during this time

Another thing separate the problem glass with clear or other colour

MayB baffle the pot 2 reduce direct radiation

Another thing is the last bit of glass will B realy heated n long in the pot

MayB remove this last bit of glass take pot out early

I think U R working in the realms of the hot glass worker ( blower ) n they get a lot of fun weirdness going on in there melts

Brian

[charlie holden]

First, I should say that I use Bullseye so I can't speak directly to how Spectrum behaves. It seems especially strange that you should be having most of your problems with clear, since that should be the base glass that everything is matched to, as far as compatibility is concerened.

I don't believe the theory that compatibility can change much at the times and temperatures you are using. I've done pot melts at 1700 for eight hours and the glass has remained compatible. The only thing I can suggest for the problems with the clear is slow down annealing and soak at a lower temp. You should relieve stress at a lower temperature than the softening point. I don't know where the softening point is for Spectrum, but it is probably about 50 C above the annealing point.

I think that the small cracks are due to the glass eating your kiln wash. By the time the glass rolls across the surface, the kiln wash is mostly powder. There is no binder to hold it together. It gets sucked up far enough into the glass to cause little points of incompatibility. When you sandblast off the kiln wash stuck to the surface, you are putting tiny little cracks in the glass surface that run into areas of stress. If you are using silicon carbide to blast with, you may think of going to aluminum oxide instead. You should be looking at your glass through a polarimeter throughout the entire clean up process, so you can isolate the points of stress and see if they are there before you sandblast, or not.

Meanwhile, I've said it before, drip your pot melts onto bats made of plaster/silica. The plaster has enough strength left at those temperatures to keep the glass from eating the release. This is the only way I've been able to make these work consistently, and I've done melts with twenty pots running together. It is hard to keep the plaster from cracking, but you're blasting it anyway, right? Or maybe you can experiment and mix your kiln wash with some plaster.

ch

[revjerry]

Randy,
Sorry to hear about your troubles. Pot melts are great fun until things go wrong as they have for your. So, I offer a couple of suggestions which may, or may not be of value; you decide.

First, I also agree that changing compatability at high temperatures isn't realistic. COE isn't the only issue here, but COE with Spectrum is pretty good and consistent; I'd be surprised to find out that is your problem. I've done a lot of pot melts using your color mixes and have no problems with them. I think you are having another problem.

Pot melts and combings have a lot of similarities in the trouble spots; mainly, getting the crap off the back. Kiln wash, fiber paper or what ever you drip onto is going to be disturbed and tend to become a part of your melt. If it gets inside the disc, then you've got real trouble. Besides, getting that mess off the back is a real pain unless you've got a sandblaster, so my approach has been to avoid it rather than deal with it.

Two ways to do this. I often drip into a containment vessel to get a size and shape I want without having to use a tile saw. When they are the right size, I use flower pot trays as a containmet field. Kiln wash first, sprinkle a liberal coat of either talc, silica or plaster onto the still wet tray and this sprinkled coating will stay in place pretty well. The problem areas are where the side meets the bottom, so pay attention there. The final trick that saves my bacon is to put a piece of glass in the bottom of the pot.

I use either white or clear typically, cut it to the same size as the bottom of the tray, set it in place using a suction cup or the suction device on your circle cutter so you don't disturb the talc and melt on top of that. The solid piece of glass doesn't move or pick up anything on the back.

Sometimes I'll get a bit of kiln wash on the edges, but that's a walk in the park to grind off. If you do this you will avoid any crap mixing with your glass.

My firing schedule is a bit different than yours as well. So if the kiln wash isn't the problem, I'd alter your cooling to:

Process to 960 AFAP (Not flash, but 9999) Hold 1 hour per 1/4" of thickness plus one hour more if it's a big piece.

960 to 700 at 75 per hour. That allows gradual cooling through the danger area. On a large pot melt or cast I'll go as far as 600, but I'm hyper conservative about such stuff..

Between keeping contamination out of your melt and an altered cooling cycle, I believe you can beat your problems. If not, say some more to us and someone will come up with something.

Good luck,
Jerry

[jerry flanary]

Great Pictures!
It looks like compatibility problems. Is the contamination from kiln wash likely? Don't you have a clear landing pad like Revjerry suggests?
I believe that things show up after (or during!) sandblasting because to a certain degree, the stresses on the surface of the glass can act as a tempering. Then blasting erodes these areas and upsets the balance. I believe it simply hastens the inenevitable and is a good thing; I don't want to sell a piece of glass with that kind of problem.
One other thing to try: start off AFAP to 1100 and then soak for an hour or two depending on how big your kiln (Thermal Mass) is. Then run up to your melt. This should make it run up faster.
Does that white have selenium in it? That is the culprit in COE changing colors. If so and you think that is your problem then get a white w/o it. I usually don't think of white as a selenium color but I am not a chemist so have no idea...

[KellyG]

I've often wondered if leaving the clay pot in place has an effect on annealing these melts? Somehow disturbs the evenness of heating/cooling.

I've only done two melts, and both eventually cracked directly down the middle. Both were done in my smaller kiln (12" shelf max), and annealing schedule has been fine on everything else done to same size/thickness. Just makes me wonder about the pot effecting things.

...Kelly

[revjerry]

KAG,
I use them a lot and never have any problem from them in respect to annealing. In fact they work the same way any mold would. The two probllems with them is, not getting them prepeared correctly, and not understanding how thick the glass is because it's been contained.

Using these flower pot trays is a blessing, and like most blessings, they have down sides. I used to do pot melts directly on the kiln shelf and let the glass do what it wanted to do. Rarely had an annealing problem but always had a lot of crap on the back and a mis-shaped piece that took a lot of cold work; the pots cure both things.

However, you have to remember that containing the glass makes it thicker and to a degree you might not anticipate. It's important to figure out how thick that disc is going to be before you start and plan your annealing based on that. Also, I use Henry Halem's annealing concept in going down from annealing temp through about 700 degrees slowly! Typically I go at 75 an hour. In my smaller kiln, however, I'll take it as far as 400 before letting things shut off.

Just some musings about the pot melt world. It's fun to hear how others meet these problems, so don't keep your processes secret.

Jerry