The aim of this thread is to explain the annealing process, by giving the scientific facts (as accurate as I can find) & simplifying them to what lampworkers need to know.
1. Glass specifications
2. Annealing
3. Kiln design
4. Measuring temperatures
5. Glass colors & reactive glass
Glass specifications
Glass does not have a melting point (not like ice, steel, etc), as ‘solid’ glass is heated, its hardness gradually gets softer until it reaches its ‘softening temp’ (a temp that the manufacturer thinks the glass can easily bent – this is open to interpretation!!). When you keep heating, it gets to the ‘working temp’ (when the glass can easily be welded together – again subjective!!!). This lack of a (definite) melting point is why some describe glass as a “supper cooled liquid”.
Some typical soft glass specs are;
Glass can be stressed by an external force or by uneven cooling – as the glass cools it shrinks a little. The outside of thick glass cools quicker than its centre, its outside is under tension & centre of thick glass is being compressed – this is unstable glass & it can crack. The annealing temp (& above) is when the glass will release these internal pressures (tension & compression).
The strain point (& below) is when an external force is not permanently absorbed into the glass when the force is removed.
Between room temp & 300C, soft glass can thermal shock with a sudden temp change of about 90C, for boro glass this is typically 250C. I don’t have any scientific documentation for this & no body defines ‘sudden temp change’
Annealing
Annealing is heating the glass above its annealing temp (but below the strain point – so your object is not deformed), then slowly cooling it, so it does not develop any internal stresses.
Annealing process has 4 stages
1. Heating the glass to its annealing temp, but slowly so it doesn’t thermally shock the glass & crack
2. Hold at the annealing temp, until all the glass thickness is at this temp
3. Slowly cool the glass (without introducing new stress) until the temp is below strain point
4. Rapidly cool to room temp, without thermally shocking the glass
My annealing schedule for soft glass is;
Annealing schedule for Boro;
Notes:
1. Annealing time (step 2) is dependent on the thickest part of your object
Up to 6mm (0.25”) 10 min
12 mm (0.5”) 30 min
24 mm (1”) 50 min
If mixing objects with different thickness, use the longest time
2. Rate of heating/cooling are the maximum rates (you can be slower).
3. If you park objects in kiln before annealing, park temp is typically 40-60 C below annealing temp. You need to modify step 1
4. My kiln cools at a much slower rate the step 3, so I could turn the power off after step2 has completed!
5. I do not vary my schedule for different types of soft glass
The above 2 schedules are what works for me, with my kiln & its temp controller. With a different kiln, I would need to vary my schedules. Other lampworkers will have other schedules that they like.
Poorly annealed objects may crack within hours of reaching room temp, or it could take years. They can also crack with a weaker external force than the glass would normally withstand.
Glass that is under stress will slightly alter its refractive index (the angle light is bent when it passes through glass, eg prism). This can be seen in polarized light (Tips,Techn…., Polariscope, by Cosmo).
Flame annealing
Much has been said about flame annealing & is very controversial, but it works for me.
If you heat the object above the annealing temp & cool it slowly so not to introduce internal stresses, flame annealing works.
My method;
1. Heat to a very dull red (typically 600C or 1100F – ie just below softening point), use a cold flame & out at the end of the visible flame, let the head soak through the whole object, but
do not melt it.
2. If the object doesn’t have small hole, etc, I turn off oxygen/air & coat it with a layer of soot – this slows the cooling more.
3. Put it between a thermal blanket (or us vermiculite) – it should take 30+ min to cool to room temp.
When cold wipe off any soot.
I will batch kiln anneal later. When I re-started lampworking 4 years ago (after a very long break, I didn’t have a kiln for 2 years – I now kiln anneal anything I sell.
This works well for thin objects, but is dodgy with thickness of 12mm (0.5”) & above. I learnt this method in the early 1970’s & still have soft glass objects from then - only flame annealed.