Jen
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Faraday's Heavy Glass
I'd like to attempt to make Faraday's 'heavy glass', which he used for electromagnetism work. (The Royal Society still holds some
original pieces which he used to show how he could change the polarisation of light with an electromagnet. )
It should also be easier to make because it fuses at a much lower temperature (mentioned in the book I have only as 'red heat' than most other glasses.
I can't imagine how I'll even reach those temperatures, probably not in my current accomodation, as I can't see my college looking
favourably on me building a furnace in the back yard. At any rate, the recipe I have (from 1870) is:
Protoxide of lead - 104 (PbO?)
Silicate of lead - 24 (Presumably Lead(II) Silicate)
Dry boracic acid - 25 (Boric acid, I presume)
From what I can tell these are ratios by weight.
Has anyone else ever played around with the chemistry of glass or attempted to make something like this? I guess I'm going to have to buy, beg,
borrow, steal or mostly likely attempt to make a furnace at some point if I want to have a go.
ps. The book also mentioned etching glass using gaseous HF. Yeoch.
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mick
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If you are doing anything with glass it should be annealed. Heat it up to red heat and control the cooling, it takes a long time if you are doing it
by hand, but it can be done.
mick
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vulture
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Take a look at some of our furnace topics for ideas about the furnace.
However, if you only need small quantities you could get away with a MAPP or acetylene torch and some good insulation.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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I am a fish
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If achieving high temperatures is problematic, investigating changes to the composition may be worthwhile. The given ratios probably aren't the
eutectic ratios, and so by tweaking them, it may be possible to lower the required temperature.
Also, am I right in thinking that the long annealing time is to allow the components of the glass to diffuse into an homogenous mixture? If so, could
the required time be reduced by mixing the components more intimately? In particular, could the mixture be prepared by precipitation from solution,
so that the components are mixed at a chemical level?
For example, to mix lead oxide with lead silicate you could:
1. Prepare a mixed solution of sodium carbonate and sodium silicate.
2. Add lead acetate solution, and filter out the resulting mixed precipitate of lead silicate and lead carbonate.
3. Heat the mixture, to convert the lead carbonate into lead oxide.
1f `/0u (4|\\| |234d 7|-|15, `/0u |234||`/ |\\|33d 70 937 0u7 /\\/\\0|23.
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rift valley
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Is a MAPP torch hotter then a propane torch? If so by how much?
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neutrino
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The purpose of annealing is to relieve the internal stresses induced in glass by its cooling and solidification by holding the glass at a teperature
below its softening point for a period of time. By the time it comes to annealing, the glass should already be homogenous.
MAPP gas burns a little hotter than propane, but releases a good deal more energy. Propane has a max. flame temp of 2800*C and releases
10.5Mj/m<sup>3</sup>, while MAPP gas has a max. flame temp of 2900*C and releases 15.5Mj/m<sup>3</sup>.
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Cyrus
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What, no furnaces allowed at your college!? 
If there is an enthusiastic science dept. there, they might allow it, but it depends on size, etc.
Besides, you don't have to tell them. A good charcoal furnace should have no
smoke or smell, and no visible flames, just the sound of a small blower.
Even though MAPP or propane might work, I wouldn't try, wouldn't the glass crack because of uneven cooling? I have no hard data to back
this up for this type of glass, but I do know that glassblowers let their pieces cool very slowly, over many hours, which is only achievable with a
furnace or something hot with a large thermal mass to let the glass cool down in.
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mick
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I think the ideal glass blowers furnance for good quality glass has a step wise cooling program. So long at red heat and then a few hours so many
degrees lower etc. I did have some bargain good quality pyrex glassware which must have been stored in a cold warehouse for years because it all
started cracking. If it had been anealed it might have been as good as new.
From what I remember, when I was anealing by hand anything bigger than a basic join or a t-piece,such as an side joint on a quickfit flask, (I was
using piped gas + oxygen)), if you got the joint right then it was a matter of sitting there and spreading the red heat further and further from the
work and then finally heat the whole lot up with a large flame above the smokey point, the longer the better, and put it down gently to cool to room
temp.
There is an experiment in glass blowing where you make a gob af glass on a long strand and it is very hard to break the gob with a hammer but if you
break the strand by hand the gob shatters. It is all down to where the stress goes. I used to make the opposite, this is true, the bit that fell of on
its own I tried to break, but it was rock solid.
If you are mixing different brands or types of glass they could have different expansion rates with temperature.
I think glass gets the opposite of work hardened, the more regularly you heat it up and cool it down the less chance of it becoming brittle.
I do not think good quality glass should be stored cold for long periods.
mick
Typo
[Edited on 26-9-2004 by mick]
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unionised
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I thought that repeatedly heating and cooling glass was likely to make it devitrify.
I also know that, as a chemist, I keep glass stuff in the fridges and freezers for years with no problems.
Does your fridge have a light bulb in it?.
That experiment with the shattering glass drops is, iirc, called "Prince Ruperts drop" if anyone wants to google it for more details.
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Oxydro
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I remember the Prince Rupert Drops thing from a book I read a long time ago... I believe they are made by dropping molten glass into water. (sounds
risky!)
I would personally go with an electric furnace. It can easily be scaled to whatever size you want, it produces no combustion products (other than from
what you put in it), and it can be controlled finely so you can anneal the glass properly, which can take many hours. Insulate it well enough and you
might even be able to do it all inside, so long as you vent any gas etc outside. I still would recommend going outside for extra safety, but the
option is at least available with electricity.
I've had no luck trying to find the composition of Faraday's heavy glass (although I've only googled for it so far), could anyone point
me to something useful?
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