Garaging and annealing, two different aspects of the same beast

You already know that your kiln is a multipurpose device. You can use it (depending on the model and capabilities) for annealing, slumping, fusing, maybe making PMC.

Garaging is the practice of keeping your work hot, above the strain point, but below the annealing point. Any time you put a finished bead or pendant in the kiln, but before you start the annealing cycle, you are ‘garaging’.

Garaging is also the practice of keeping parts hot prior to assembling a finished piece from those parts.

So, what temperature should I garage my work at? It all depends on what type of glass you are working with. My particular preference is to garage at the halfway point between the annealing temperature and the strain point of the glass. (Remember that the strain point is the temperature at which the molecules of glass stop moving!) For example: Moretti/Effetre glass has a 960 F annealing temp and an 850 Strain point. The temperature that I would recommend for garaging Moretti/Effetre is 905 F. For borosilicate glass, the annealing temp is 1050 F and the strain point is 950, so your garaging temp would be 1000 F.

And now, the Great Annealing Debate!

Remember that annealing is the process of keeping the glass hot enough so that the molecules can “straighten out” after being mixed and allowing the strain to release. For Moretti/Effetre, the “ideal” temperature is 960 F, for borosilicate it is 1050 F. But is that the only temperature? No it is not. (And once again, I’m going to horrify some folks.)

Glass will anneal at high temperatures, quicker. This is basic physics. But, there are also drawbacks to this practice. If the temperature is too high, the glass will either slump (deform) or simply melt. Additionally, depending on the chemistry of the glass, you may experience loss of color, bleaching of color, or other unexpected results.

Glass will also anneal at lower temperatures (above the strain point though), albeit for longer periods of time. This is a process known in the precision optics field as fine annealing. It is not uncommon to hold at the desired annealing temperature for a week or more…and in some cases, for very large pieces, the process can take literally months!

So, what are the ideal temperature curves to anneal glass at? I strongly recommend that you look at the materials supplied by the manufacturer of the glass and follow thier guidelines. But, if you are a curious person and want to experiment, here is a guideline you can follow on your own:

Hold at annealing temperature 1 hour for every 1/4 inch of thickness.

Drop to strain point temperature (usually around 100 degrees below annealing temperature) over the same time period you annealed for. For example: if you held at the annealing temperature for 2 hours, then use a 2 hour time period to drop to the strain point temperature.

Drop to room temperature over 4 hours (or as fast as your kiln will allow). Do not open the kiln door to speed cooling!!).

There are other factors involved, and I’m grateful to Northstar Glassworks for their wonderful inforation!

Here are the two annealing cycles I routinely use for my borosilicate work:

Cycle one: Non-striking colors (basically no silver content glass)

Garage at 1000 F, hold 1000 F for 30 minutes after last piece has been put in kiln
Ramp to 1050 over 10 minutes, hold 1050 for 2 hours
Ramp down to 950 over 2 hours, hold for 10 minutes (the 10 minute hold allows the kiln to stabilize at the strain point)
Ramp down to 74 over 4 hours, end program at that point.

Cycle two: striking colors (all silver content glass)

Garage at 1000 F, hold 1000 F for 30 minutes after last piece has been put in kiln
Ramp to 1225 over 10 minutes, hold for 10 minutes
Ramp to 1050 over 10 minutes, hold 1050 for 2 hours
Ramp down to 950 over 2 hours, hold for 10 minutes (the 10 minute hold allows the kiln to stabilize at the strain point)
Ramp down to 74 over 4 hours, end program at that point.

Annealing is an ineaxact science, perhaps more art than science because of the number of variables involved. I strongly recommend that everyone play around with their annealing program and use one that gives them the best results. There is no one single program that is gonig to work for everyone all the time.

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8 thoughts on “Garaging and annealing, two different aspects of the same beast

  1. So will it do Bad Things to your glass colors if you anneal your non-striking colors on the striking colors cycle? I ask because I rarely use only one or the other during a torch session.

  2. It possibly can. Glass Alchemy states that holding the cadmium colors (the opal/crayon colors) at higher than 1050 for long periods of time can cause them to burn out, but if you are using a fairly short strike cycle (like my 30 minute example above), I don’t see that it will cause a problem (or at least a major problem).

  3. Hi Mike,

    I have few questions about borosilicate and I was hoping you can help me out here. Let me first tell you what my design is about. I’m planning on firering in a kiln (not by myself) few bowls using borosilicate glass, then use them to cook quick food on the fire (tips of the fire). I’ve got the glass sheets and I’ve done some research about boro. Here are my questions:
    1) I’ve looked the annealing cycle for borosilicate and for the thikness of glass I have ( .65mm ~ .2554″ ) they suggest the total time 2.25 h. Isn’t that too short? I was under the impression that boro has to cool down for as long as possible. Is it ok to add time to cooling cycle?
    2) I do have to reach the softening point in order to shape the glass to the mold, correct?
    3) The fire tips temperature is about 608F and the strain point is about 950F. Is the glass going to be safe that way or am I creating stress on it by bringing it to the 600ish F temperature and colling down on and off?
    4)I would like to incorporate 1 tranparent color in the clear borosilicate glass. Like a dot on a bottom of a bowl or something like that. I know that different colors anneal at different temperatures. Is there any color that have annealing cycle close to clear? Am I risking strength of the glass by mixing two colors together?

    Mike, I know it is a lot of questions, but I will be happy for any answer you could give me.
    Thank you!

    • I think you are going to be unhappy with the results — borosilicate glass doesn’t like to be slump and/or kiln formed. I’m not sure what the problem is but the result is devitrification on the surface of the glass. You can certainly give it a shot to see if it works, but most everyone I’ve talked to about this has said it isn’t worth the trouble. Soft glass sheet gives much better results and is a proven material for kiln slumping and forming. Boro isn’t (yet).

      To answer your questions:
      1) No. That’s the time at the annealing temperature of 1050. 2.25 hours is actually a bit on the long side IMO.

      2) Yes. You have to get to the softening point and HOLD it there until the piece slumps into your mold.

      3) A “proper” annealing cycle is to bring the piece to the annealing temperature (in the case of boro, about 1050F) and hold it there for a period of time to allow all the stress to “bleed off”. There is still some movement of the glass molecules at this point, and this is good. Hold for at least an hour or as the information suggests, about 2.25 hours. The drop your temperature to the strain point. The strain point is where the glass molecules stop moving and any remaining strain will be ‘locked’ into the glass. I suggest a 1 hour drop from annealing to strain point, then about a 15 minute hold at the strain point to allow the kiln controller to catch up to the actual kiln temperature (there is usually some lag either way and you can’t harm the glass by holding at the strain point). Then drop your temp 100 degrees per hour until you get to room temp.

      This is a very conservative schedule and “should” result in zero breakage, but your milage of course may vary.

      4) Not for boro. Pretty much all of the colors will anneal at the same temp.

      Good luck to you!!

      • Mike,
        That is a great help!Thank you.
        After checking what devitrification means, if it happens in my project I may just “pretend” it is a deliberate artistic technigue 😉
        1)Do you suspect smaller pieces (4″ round plate) would be more prone to devitrification then bigger piece or it doesn’t matter?
        2)If I cook with this glass over the fire do I still have to cool it slowly to room temperature or it doesn’t matter (the fire temp is below strain point)
        Thanks!

  4. Devit will happen anytime the glass “moves”. I suspect that you will see it regardless of the size of the piece. I’m thinking that it will be more apparent in the bottom of the bowl around the “corner” where the base changes into the sides.

    I think you will be surprised at how hot a fire actually is… although the temp should be below the melting point of the glass, you may see some deforming of the glass unless you have it full of cooking food (which will absorb the heat) – in the same way that you can put a paper cup full of water in a fire and it won’t burn until the water evaporates.

    Personally speaking, I wouldn’t go to all this work to make a piece of art and then use it on a fire. I’d use it on the stove perhaps…but then again, even when the Pyrex brand of glass was made of borosilicate (it is soda lime these days), they always had a notice on it that said something to the effect of not using it on an open flame.

  5. Thank you very much. I have read a lot of materials & books on this. You were able to help me with my annealing issues. Thank you again!

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