Two slightly random queries...

[Lucas]

Okay, first one - I read somewhere on here (can't find where now) that someone (sorry don't know who!) opens their kiln once it gets to about a hundred degrees - my kiln takes ages to cool and I've been forcing myself to wait til it gets to 20 degrees (C) til I open it, but that last bit takes hours and hours - what temp do people feel secure opening their kilns without worrying about thermal shock etc?

Second query - again a fusing question - I get the tack fuse, and the full fuse, but what is there in between?  I mean, I keep finding references to whether a tack fuse is 'agressive' or not, and I've seen something else about a 'soft fuse'.  I think of a tack fuse as being literally tacking two pieces together, so what kind of firing schedule would I need to get the rounded corners etc but not completely sunk one layer into another...does anyone know what I mean?  (bullseye BTW)



Lucas

[Zeldazog]

If I am around, I crack my kiln open a little bit at around 100 degrees, then at roughly every ten degree drop open it a bit more, until fully open at around 50 degrees.  I then take work out when it's cool enough to handle.

Although I don't tend to do this so much now the kiln isn't at home in the kitchen, I am not usually around at the time it hits the temperatures.


As for the second question, I can't say I have heard the term aggressive or soft fuse, but I think I get what you mean.  The unique nature/structure of glass means that it doesn't turn from solid to liquid at one set temperature.  It softens as it gets hotter, and it will do things across a range of temperatures.

So, there is a lowest temperature it will tack fuse at, where you will get a squared off fuse with only slightly softened edges, basically no more than a fire polish really.  At the upper end of the tack fusing range, it will tack and round off more but still not all sink into one layer.

There is no exact temperature that anybody can say, as it is a combination of working temperature as well as hold, let alone individual kilns.  Some glasses soften earlier than others too.

This is a really good tip sheet from Bullseye that explains the range of temperatures at which different processes will happen - Bullseye Tip Sheet 04
.  There are lots of other useful tip sheets on that site too.

Basically, it's experiment time Lucas.  If the temperature you tack fused at didn't round the glass off enough, take it up another ten degrees or so, and try again, until you get the result you want. 

[Rikki]

I have been opening my kiln at around 125 degrees and then take things out once they are cool enough to handle without (too much) burning!! Sometime I get them out an put them on a ceramic tile to cool. I have never had any problems with anything breaking. I started off in a microwave kiln pot thing where you had to wait 1/2 hour before opening it. I managed to wait the 30 mins but no longer and stuff was often still VERY hot but even then it never broke. I would leave the lid off until cool enough to hold. So, unless I've been very lucky it seems to be OK once you get around 100 degrees-ish.
I have been producing glass that is not quite fully fused in that it is rounded but you can still see the two pieces of glass as separate pieces. This was by accident whilst trying out fusing schedules, I ramped it up to 810 degrees, no hold and then immediately cooled. Obviously it would be different in different kilns.
   Hope this rambling is of some help!

[Lucas]

Thanks guys!
As you say, time to experiment!
L

[stuwaudby]

Glass anneals above around 350 Degrees. Annealing means that the molecules in the glass are able to move over each other to releive internal stresses. Below 350 the molecules positions are effectivly fixed (they still move a bit but slowly).

What this means is that the cooling rate below 350 will not affect the internal structure of the glass and will not impact the anneal. What you do have to worry about is causing stress by allowing one part of the glass to cool faster than the other. The worst scenario is where the outer skin of a piece cools faster than the inside. The cooler outer then shrinks and squeezes the hot inner, the outer is pulled apart which results in a crack at the surface. Once a crack has formed all the stress is focussed at the tip of the crack and causes it to spread rapidly.

All this means that there are a number of factors which will determine the risk of cracking:
Difference between Piece temperature and Air Temperature
Air movement (A draft will cause mroe rapid cooling)
Piece size
Piece thickness
Glass Mixture (different colours radiate heat differently)


Rather use an absolute temperature as a safe point you should use the difference between kiln and air. i.e. on a hot summers day in my conservatory the difference  between my piece at 100 and the air at 40 is only 60 degrees. In the winter it is nearly double at 100 Degrees.

My big kiln cools very slowly. If I am in a rush I regularly lift the lid or remove the vent to allow it to cool faster under 350 DegC. For small pieces opening up at 100 shouldn't be a problem, I wont risk a larger piece. For safety you should wait till they have cooled below 50 before touching them.

[Zeldazog]

Great explanation Stu.

I was going to say the same about the risks involved dependent on size and thickness of kilns.  When Rikki says she opened the microwave kiln at such high temperatures, those pieces are very small.  Personally, I wouldn't risk it on a 12inch plate.  Very good point about ambient temperature too.

As for experimentation Lucas, I know I probably sound like a stuck record when I recommend this, which I seem to be saying a lot lately!

But you really do learn so much more about fusing glass when you "do it and see what happens" rather than following a set of instructions. 

Aside from the differences in glass, in kilns, in conditions for everybody, if you understand WHY you alter something and what effect it has, you are in a better position in future to understand HOW to alter something to get the desired result.

The teacher who showed me glass fusing, when asked "what will happen if I do this" used to say "Try it and see".  I feel that I learned so much this way and understand glass fusing, and kilns, so much better.

[Lucas]

All makes sense - thank you!

[Flyingcheesetoastie]

Everything technical has been covered well already but I just wanted to add to everyone opening their kilns and venting them above the room remperature they are in....is it worth it?  You have spent your time, energy and money cutting, preparing molds, loading the kiln, programming and waiting.  What is another few hours just to let the kiln do what it's sposed to do?  I know I've been guilty of doing it myself but if it's a piece of work I've invested any significant time or resource in, then I just leave it there till it's cooled down!  Remember even if you have got it out the kiln, juggled it round like a hot potato and it hasn't cracked, doesn't mean you haven't introduced stress into a piece doing that.

Be nice to your glassy creations!

R x