what temp to soften glass so gravity makes it flow

Depends on the bottle. Gray Goose bottles are really thick at the bottom and need to be cooled really slowly. take 'em up to about 1550 (every kiln is different) at 500degrees/hour, then drop 'em to 1200 (again, 500 degrees/hr, hold for a couple hours, drop to 900, hold for a couple hours, then shut 'er down. Whole thing takes about 24- 36 hours until shut off.m

Can't you bring it up to 1500 or so and just flatten the bottle with a piece of steel plate with a handle? It would save many hours of waiting.

1550 seems pretty high if your goal is just to slump the bottle. Does bottle glass really need a temp that much higher than, say, float, which full fuses around 1560, tack fuses around 1430, and slumps somewhere below that? Or are you going for the "slump really fast, vent really fast, hope you don't get a phone call at the wrong time" schedule?

Go up about 500/hr to around 1500F. Check on the bottle every so often after you hit 1400. When soft the bottle will flatten in a matter of minutes. Drop as fast as possible to around 960 and soak for about an hour. Bring to room temp over the next 3 hours.

The hold at 1200 will serve no purpose. Also, above 1000 you can move that temp as fast as you want.

When you reach temp the bottle will flatten in a matter of minutes.

My bottle slumping days are over, I just get the vodka bottles empty for the slumpers. I'll have to get a Grey Goose bottle just to see if I can thermally shock it above 1000F.

I have a small slab in right now. 8"x12"x1". I'll have it out in slightly less than 24 hours.

I'd try it to support my habit but I doubt anybody would by Kamchatka bottles.

I do bottles all the time. If I go to 1,500 in my kiln I get a puddle that gives little hint of once having been a bottle. I do have a controller and would start by holding it at 1,350 for 30 min. If not slumped enough I'd try

1,400. I agree that a soak at 1,200 does nothing. When I have glass that's 1/2" think I slow down the annealing, from about 950 to 700, so that a whole firing takes about 24 hours.

John Bassett

I see them at craft fairs with the original label glued back on the now flattened bottle, and used as ashtrays, or wind chimes. They have so many different ones that I always thought they were fired to 1200-1400 and then flattened mechanically all at the same time?

It is probably the case that someone built a 4x4 foot, not very deep slumping kiln and does a couple of dozen bottles at a time, redoing those that don't slump enough. They may also have a mold for part of the shape. No matter the choice of glass the basic pattern is: Find the sag point (where a long thin piece of glass slowly sags of its own weight when supported at ends.) Calc the annealing point at 50C/90F below the sag point for practical purposes Heat the glass up slowly enough that it does not crack going up until past the sag point. Heat as fast as possible to the desired slump, observing when possible and safe. Crash cool from the slump temp to the annealing temp to stop the movement of the glass and prevent devitrification (white patterns on the glass from decomposition.) This is normally done by opening the lid several times. Soak at the annealing point to bring it all to the same temp and little strain. Slowly cool the glass from the annealing point to the strain point over several hours depending on thickness of the thickest piece. For most soft glass, the strain point can be taken as 300F below the annealing point. Continue cooling as fast as the kiln will drop to near room temp.

I have thought it would be neat to have a couple of annealer/ovens handy so one could be holding the glass for annealing or fusing and the other could be running from annealing to slumping and back down again. Then the worker could stack the 900F slumped glass in the annealer, move new bottles/forms at 900F to the slumping oven, etc. Of course, the real problem is that most artistic fusing can't be handled this way as it is a balancing act of fragile proportions.

The information on establishing the high end annealing temperature of an unknown glass is not quite accurate. I would recommend the following.

You will need a rod approx.18" long and about the thickness of a pencil fabricated from the glass you will be slumping. This rod is suspended at one end between two IFB's. Raise the temperature as fast as you want to as a rod of that dimension will be able to take a quick temperature rise. Since we're talking soda lime glass we can assume it will anneal in the mid to high

900's but we don't know. Raise the temperature to about 1000 deg. F. and hold it there for 1 hr. Check the rod if it is still straight raise the temperature another 50 deg. and hold for an hour. If it is not deflected at all raise it another 50 deg. and hold an hr. When you see a slight deflection of the rod you have found the softening point of that glass. Check the temperature reading on your pyrometer. The high end annealing point is 50 deg. F or 28 deg. C below the temperature reading on your pyrometer not 90 deg F. The temperature where the glass has deflected is called the softening point. You can get into trouble if you anneal 90 deg. below the softening point. One cannot assume the strain point to be 300 deg. below the annealing point as one has to mechanically measure that point and that can only be done at a testing lab that has that type of equipment. In reality strain points are really only necessary when doing thick castings and not necessary for what most do on this board.

Mike Firth

I think that you mean suspended between two IFBs, one at each end of the rod, Henry.

Or are you using the old softening point determination method, where you follow the position of the end of the vertically-suspended rod or fibre with a travelling microscope as the temperature rises?

Gee, Henry, thanks for the info, except I thought that the last time you corrected me, you were the one that said 50C (90F) not 50F. In any case, isn't a 50F step a little big, especially if you are going to back off 50F for annealing, and hold for an hour a bit long?

If I'm trying to establish this sort of thing I'm moving in 10's.

Go up about 500/hr to around 1500F>

all good tips -- but many mention of Grey Goose bottles

--- what about the enamel on them?????? if it is low temp enamel -- it's going to go bye bye at those temps.....

Cheryl DRAGON BEADS Flameworked beads and glass