inside out cracking

[Simeon]

I tried doing a search before posting this thread, but im not so good at asking search engines the right questions so i figured lets see what i can get from the forum without anoying the vets.

Im sure this gets asked alot, why is my iso cracking, its all 3-d and stuff? Answers: you probably didnt melt it in all the way even though you got some 3D in your color. or you didnt even come close to melting it in. anyway, this is the most common problem.

So lately i have had some inside out pieces crack on me after going through the kiln and yes the color is fully melted. The only things i can think of are this
A: I took everything out of the kiln around 200 degrees, which normally doesnt matter but it has been -20s outside which makes for a very cold shop in the morning, probably in the low 30s when i get in. CHilli, i know.

B: I have heard that some colors no likey heavy encasement, i.e. exotic green, the one common factor in all the pieces that are cracking.

I am leaning more toward B because all of my stuff with color on the surface is fine, a little thinner as well but still fine. ANy other ideas on what is going wrong. I tried to repair the ones that cracked so if they dont come out this time i will post some pics. But one thing im sure of is that the color is fully melted. Lost too many a good one to let some stupid dot f-me in the A. Thanks for any info

[standard28]

Probably B, color glass has metal in it and has a different co-expansion. Whats happening is when it's cooling off the different coe's are pulling against each other until it (stress) relieves itself and separates. Its alot of trial and error of mixing colors especially different companies. They all try to keep the coe around 33 but all it takes is a little difference to stress out the glass and have it crack. With inside out the last part to cool is inside. When its on the outside the color tends to cool off with the clear allowing it to stay together...at least this is what I think, but to each his own.

[Slimy-E]

If it is the color then it is a network of spider web cracks, not a crack here or there it is shattered but often holds together.

you will know by the way it crinkle cracks all around the problem color.

How melted in? whats your kiln cycle temps and times? What colors? pics would help!

[Simeon]

im having trouble with exotic green, and aurora. I wouldn't call the cracks a web really but they do start around the color. The walls of the piece are at least 1/4" thick, everything is melted smooth on the inside, no rasied color or "tits" .

As for my annealing cycle, everything soaks for 4 hrs, then starts the decline, holding above the freezing point(just above 980 i think) for an hour and then down under the freezing point (i want to say its 900) and holding again for an hour, i over kill my soaks so that when i am doing thicker work it doesnt get rushed. I got the schedule for my ramp out of the "Essential Lampworking" book, i cant remember exactly how it goes with out looking it up but i know its legit.

Should i have a different ramp for inside out work?

i may post pics latter today, still waiting for the kiln to get to a reasonable temp. if they crack again i will deffinately post some pics.

[standard28]

1065 anneal 30 mins...ramp up full ramp down full. 980 won't anneal, even at an hour.

[Deez]

what is the freezing point

[Simeon]

thats where i hold first 1060....980 ish, cant remeber what the exact temp is, is the next step down. The wierd thing is nothing else inside out is giving me a problem so i dont think its my ramp but i will look it up.

what are you using for a ramp?

Thanks for responding!

[Simeon]

"freezing point" = temp at which glass is no longer in a "liquid state", above 980 for boro , and i think 960 for 96 COE

[HDGlassArt]

Dumb question... Did you use any china glass?

[Simeon]

Nope, using Corning Pyrex, caught the suff on sale, wonderful glass

oh and the book i used to set my annealing ramp came out of "contemporary lampworking" not essential lampworking, so i think it is probably a solid ramp. Wish i had a copy of it, but my buddy borrowed it and has yet to return it.

[Slimy-E]

ok, both green exotic and arora have been known to be problem colors heavy exotics have lots of metals in them they do not generaly like to be reduced at all before incasing.

I run my kiln at around 1080 when working then got to a 40 min annealing at 1050 then a ramp down to 900-just under stress point(not freezing temp), hold for 15 or so, ramp to 700(the temp when glass stress from cooling is now temporary and cooling speed can double) , hold about 20 mins, then crash

[p.j.]

ok, both green exotic and arora have been known to be problem colors heavy exotics have lots of metals in them they do not generaly like to be reduced at all before incasing.



^^^the reducing is probably leading to the coe shift and and cracking, if no others pieces were affected

[Simeon]

right on, i was just looking at the northstar annealing chart and 960 is listed as the strain point, i have also heard it reffered to as freezing point, but strain point is a much more concise picture of what the temp represents.

I can see where a higher garage temp would be good, keep from flashing em with cold air when im going in and out, but i am pretty sure i have just over reduced the piss out of the colors, i screwed up at step one, reduced em pulling the stringers, 2. reduced while laying em down, 3. reduced melting in. I am guessing with colors like these i will be better off reducing only right before i close the tube down, and maybe going a little lighter on the wall thickness or maybe just not at all. Live and learn. Good news is all my other inside out work came out nice.:bouncy:

Thanks for the posts Standard 28, Slimy-E and p.j.

Simeon