Lampwork Etc.
 
TrueDesign

LE Live Chat

Enter Live Chat

No users in chat


Jelveh Designs - Glass Beads Torched One-by-One

Glacial Art Glass


 

Go Back   Lampwork Etc. > Library > Tips, Techniques, and Questions

Tips, Techniques, and Questions -- Technical questions or tips

Reply
 
Thread Tools
  #1  
Old 2022-03-19, 11:17pm
phentron phentron is offline
Member
 
Join Date: Jan 12, 2022
Location: country Victoria, Australia
Posts: 30
Default Annealing, Glass Specs & Kilns

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.
Reply With Quote
  #2  
Old 2022-03-19, 11:25pm
phentron phentron is offline
Member
 
Join Date: Jan 12, 2022
Location: country Victoria, Australia
Posts: 30
Default

Kiln design
Kiln design will affect annealing & thus your annealing schedule.
The larger the kiln, greater the potential for larger temp gradient within the kiln.
Placement of heating element(s), one element at top of kiln is the worst place. 5 elements is the best - 4 sides (including door) & roof. Side element should nearly cover all the wall.
Power of heating element needs to be big enough to achieve your heating rate for your kiln size, but one too large will give excessive temp oscillations.
Thermocouple placement, at the top is bad – the rest of the kiln is much cooler, the bottom is better, but not close to heating element – oscillating temps.

Temp controller
3 common types; analogy (needle indicator) every cheap, very inaccurate – can be +/- 50C
Digital (single point) – a programable one is not much more expensive & single point usually give larger temp oscillations.
Programable – most have 30+ steps, we only need 5-8 steps, it would be very nice to have a controller that saved several schedules!! Their internal setup can be configured to reduce over shoot when set temp is reached. A badly setup controller can oscillate +/- 10C, this should be +/- 2C at max. The best programable controller will proportionally reduce power to the element as it approaches the set temp (not just switch element off/on) & reduce temp over-shoot but these are very expensive (& not what lampworkers need).


Measuring temperatures
It is difficult to accurately measure temps above 500C.

All temp controllers need to be calibrated (checked for accuracy) – the temp they display can vary from the actual (real) temp at the thermocouple by +/- 10C.
50C above annealing temp is no problem (except for some reactive glass), but 10C below annealing temp???

Simple calibration method:
Some inexpensive multi meters come with thermocouples (check its accuracy in ice then boiling water, but this doesn’t guarantee accuracy above 500C). Place this thermocouple next to the one in kiln, close door & let things stabilise. You can then take your multimeter to other lampworker’s kilns to see how measurements vary.

My kiln:
200cm wide, 300cm deep, 270cm heigh (internal), large door (no mandrel flap), heating element covering back wall 2000 watt, fire brick & thermal blanket insulation 50cm thick – it takes about 45 min to cool from 560C to 300C. Single program temp controller, actual temp is 0 – 5 C above displayed temp


Glass colors & reactive glass
Colored glass is often thicker/stiffer when lampworking, thus its working temp is higher & its annealing temp is most likely higher (many glass manufacturers/suppliers don’t give this information).

I don’t have enough experience with reactive glass & will leave this for others to comment.
Reply With Quote
  #3  
Old 2022-03-20, 5:26am
echeveria's Avatar
echeveria echeveria is offline
Senior Member
 
Join Date: Aug 31, 2006
Location: Knoxville, TN
Posts: 2,088
Default

Effetre lists an annealing temp of 878F on their website
__________________
Kathy
Reply With Quote
  #4  
Old 2022-03-20, 2:55pm
phentron phentron is offline
Member
 
Join Date: Jan 12, 2022
Location: country Victoria, Australia
Posts: 30
Default

Thanks Kathy,
878F converts to 470C, my data base (compiled many years ago) gives a range 493 - 520C, I am therefore in error & will correct my data base.

I don't have a reference where my figure came from. It is possible that Effetre (with the increase in science technology) re-measured the annealing temp to be the lower value. Years ago, annealing temp was not measured scientifically, but determined subjectively & difficult to tell when the glass was 'fully annealed'.

Peter
Reply With Quote
  #5  
Old 2022-03-20, 3:16pm
echeveria's Avatar
echeveria echeveria is offline
Senior Member
 
Join Date: Aug 31, 2006
Location: Knoxville, TN
Posts: 2,088
Default

Quote:
Originally Posted by phentron View Post
Thanks Kathy,
878F converts to 470C, my data base (compiled many years ago) gives a range 493 - 520C, I am therefore in error & will correct my data base.

I don't have a reference where my figure came from. It is possible that Effetre (with the increase in science technology) re-measured the annealing temp to be the lower value. Years ago, annealing temp was not measured scientifically, but determined subjectively & difficult to tell when the glass was 'fully annealed'.

Peter
I think there is a lot of info out there that is wrong, but perpetuated in print and person to person. Bullseye also has a tech paper on annealing their glass, but I don’t have it handy. Double Helix recommends temps lower than 920 to retain effects in their 104 silver glass. I like to try to go to the mfg source now that I know the “conventional wisdom” is not always right! Or at least not right any longer.
__________________
Kathy
Reply With Quote
  #6  
Old 2022-03-21, 8:06am
rcktscientist rcktscientist is offline
Member
 
Join Date: Jun 19, 2021
Location: los angeles
Posts: 60
Default

An important point that many people forget about is that annealing temperatures/schedules generally refer to glass that is cooled on all sides. So, a closed, hollow form or an object sitting on the kiln floor needs to be calculated differently. I have used a rule of thumb that doubles the thickness of any part not being cooled on all sides with success.
Reply With Quote
  #7  
Old 2022-03-24, 2:47pm
phentron phentron is offline
Member
 
Join Date: Jan 12, 2022
Location: country Victoria, Australia
Posts: 30
Default

2 more comments for this thread

Some kilns (probably make for pottery firing) have a vent – this should be closed for annealing. If this was mine, I would also pack the vent with some fibre blanket material.

A friend (pro glass artist & often makes coloured boro from rare materials) told me his method for determining annealing temp for unknown glass;
Suspend on blocks, both ends of glass rod (small diameter, the longer the better) in the kiln.
Very slowly increase temp until the rod just starts to bend, this gives you the temp just above annealing temp.
If you know the annealing temp of glass, this method can be used to check a kiln’s temp.

Peter
Reply With Quote
  #8  
Old 2022-04-04, 3:15am
phentron phentron is offline
Member
 
Join Date: Jan 12, 2022
Location: country Victoria, Australia
Posts: 30
Default Lead crystal & coloured glass

Much has been said about safety of using & lampworking lead crystal, but many of the metal oxides used to colour glass are just as poisonous as the lead. In solid glass, the lead & other heavy metals are strongly bonded in the glass matrix (the only real problem is with long time storage of alcohol in decanters & possibly food in coloured or lead glass & the heavy metals leaching out – there is no health problem in using these containers for short periods, ie 1 day).

Note: crystal glasses & decanters can now be made with no lead in them.

When flame working coloured glass or lead crystal, you must have a good exhaust system to protect the lampworker.
A very serious health problem is what happens to the exhaust gases (I live in the country & don’t have water supplied to my area, so I collect rain water off the roof, my exhaust doesn’t go through the roof, it goes out through the wall). If I had young children, I would not let them play near the exhaust.

Note: one of my degrees is analytical / industrial chemist, years ago I worked for (Vic, Australia) EPA (Environment Protection Authority), back then & probably still is, a requirement to register the exhaust from commercial glass works & also have the exhaust gases tested for heavy metals, etc.

Peter
Reply With Quote
  #9  
Old 2022-05-11, 7:30pm
phentron phentron is offline
Member
 
Join Date: Jan 12, 2022
Location: country Victoria, Australia
Posts: 30
Default A simple way to Test Exhaust system

A smoke test is the best way to test the efficiency of your exhaust system. Ducting specialists use ‘canned smoke’, but cigarette smoke or even smouldering paper can be used. Sit at your bench, with torch lit (these will influence air flow), there should be smoke movement towards the exhaust, particularly near your face & around the edges of the bench (you don’t want any of the heavy metal fumes ‘falling’ off the bench into your room). Also, you must have an air intake into your room or you will run out of suitable ‘breathing air’ (this will also affect the air flow test).

Less effective than the smoke test is to use a lit candle to detect air flow. The candle flame should have a slight have a slight leaning towards the exhaust intake. If the candle flame stays vertical, the exhaust is not enough. If the candle flame is sucked horizontal or blown out, you exhaust is far too strong (& probably too noisy!).

One area that is often ignored by lampworkers is what happens to the exhaust gasses after if leaves your room. In Australia, commercial glass blowing & lampworking premises should be registered with the EPA (Environment Protection Authority) & the exhaust gasses tested for heavy metals (& temperature) – these may need a filtering system.

For hobby lampworkers, do not have the exhaust gasses going near children’s play areas or near air conditioning units or open widows (or over the neighbours’ fence!). My exhaust goes through the wall, not through the roof – collecting rain water is my only water supply!!!! Also, your air intake to the room must not suck in any of the exhaust fumes (ideally it should come from the other side of the building).

Peter
Reply With Quote
  #10  
Old 2022-06-17, 6:04pm
phentron phentron is offline
Member
 
Join Date: Jan 12, 2022
Location: country Victoria, Australia
Posts: 30
Default Additional kiln information

The information about kilns (above) was based on my type of glass working (& annealing off mandril sculptures, etc).

When the kiln door is opened, the electricity must be turned off to the heating element.
If the kiln has a mandril flap, this may not happen. You don’t want the mandril to be able to touch the heating element (electrocution!), thus the heating element probably should only be on the roof of the kiln (& accept some oscillation in temps).

Kilns designed for slumping often have just a roof element, to let the heat ‘radiate’ evenly down to the plates. etc.

Peter
Reply With Quote
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump




All times are GMT -7. The time now is 8:32pm.


Powered by vBulletin® Version 3.7.5
Copyright ©2000 - 2022, Jelsoft Enterprises Ltd.
Your IP: 44.192.94.86