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Steven Tribe
10-Nov-2010, 12:06
Devitrification is the process when glass (a super cooled liquid phase) decides to form crystals (opaque/white) of it's oxide components.

I haven't seen or heard of this phenomenum in connection with optical lenses ever. Blooming yes, but not Devit (not my invention but the usual expression amongst glass blowers).

In connection with my rebalsaming, I had a Swift landscape meniscus under treatment from the 1890's where a single surface (new glass from Jena, perhaps) suddenly became opaque. This happened at about 100 degrees C. The appearance is crazed and slightly whitish.

Fortunately, I have a replacement achromat!

If this can happen at 100 deg. then it can probably happen at longer temperatures too. Any more cases out there?

Perhaps some of the famous cases of abandoned glass types from Jena were due to this, rather than the "too soft" and "weathered badly" excuses I have been printed?

J. Miller Adam
7-Feb-2011, 00:13
Devitrification happens, depending on the exact glass formula, more in the 1200-1400F range as the glass is cooling from its soft, semi-molten state, usually when it is held in that temp. range for too long in a kiln or furnace. People who do glass fusing & slumping in kilns have to deal with this issue. I highly doubt what happened to your lens was devitrification (growing of crystals on the surface of the cold glass). It could be a reaction to some environmental condition however...

Steven Tribe
7-Feb-2014, 02:14
This is a problem which I hoped I would never have to write about again.

Alas, I have had a new case in connection with rebalsaming an achromat.
This is nowhere near as dramatic as the last I met this, but still pretty horrible.
It occured overnight after a month of soaking, just a few days before total separation was completed!

Looking at the statistics of my DIY, my experience shows this will occur in about 2-4% of situations using the usual xylol/low temperature (60 deg. C) technique. When I started doing rebalsaming, I thought the big problem would be mechanical handling and cleaning the internal surfaces. This not the case.

I thought this might have something to do with the glass type used on certain lenses and considered the "new" Jena glass as the culprit. This has occured on 2 English achromats, one of which was produced unaltered since 1865 - so I have reason to believe that it is a Chance Brothers glass type which is sensitive when aged.

I'll get back when I have some photographs and if the same glass is involved. Post-mortems are not much fun.

Steven Tribe
7-Feb-2014, 04:25
Both the lens that devitrified were Crown lenses ( I suppose the barium oxide type). Only a single surface was involved. I enclose photos of the two lenses.
The first lens has had the worst of the erupting surface polished away - in the service of science, not in an attempt to get a useable lens again!
The second lens has much shallower decomposition and is like a graduated filter. It is more serious at the edge of the lens were the lens is thinnest. The colouring is some residual aged balsam.

Wayne Lambert
7-Feb-2014, 09:59
Steven,

Once in a previous life I was an optical mineralogist and saw several examples of devitrified natural glass such as obsidian. All glasses eventually devitrify but some glasses are as old as Cretaceous which shows how long the process can take. But it can also happen very rapidly as the volcanic glass is cooling. Viewed with a petrographic microscope in polarized light devitrified glass appears as an aggregate of tiny crystals of silicate minerals such as tridymite and feldspar. To ascertain if devitrification has occurred you might contact a university geology department and interest them in the project. For viewing with a petrographic microscope a thin section (slide) of the glass would have to be made; most larger geology departments would have the facilities to do that.

Wayne

Kevin Crisp
7-Feb-2014, 10:08
How were you separating the elements when this happened?

Drew Wiley
7-Feb-2014, 10:30
Glass also hydrates, and this eventually leads to surface spalling. So even your best lenses are likely to lose their value sometime within the next 10,000 years.
Better sell them off cheap, now, while you can.

Steven Tribe
7-Feb-2014, 11:28
How were you separating the elements when this happened?

Soaking in Xylol.

The first case happened at approximately 120 degrees in a hot air oven (no radiated heat).
Since I have never gone about 60 deg - but with another failure yesterday after 3 years of no problems!

The devitrification appeared in this last case during a soak after an oven treatment.
I can easily see both small and larger crystal flakes.

Harold_4074
7-Feb-2014, 16:25
Rather than devitrification, what you have encountered is more likely the effects of corrosion.

In "soft" glasses, particularly, condensation of moisture can definitely dissolve out alkali metal oxides to form concentrated hydroxide and carbonate solutions, which can in turn dissolve silica. The resulting corrosion products, if different in density ffrom the original glass, would quite possibly create sufficient stress to cause chipping. (If you soak laboratory glassware in hot Micro-Clean detergent long enough, the surface will delaminate like mica. Please don't ask me how I know this.....).

Curiously, the moisture/corrosion phenomenon will not occur in either very dry conditions or very wet ones, hence its association with condensation. In dry condititions, there isn't enough corrosion to matter, and in very wet ones the alkali hydroxide/carbonate is too dilute to have any effect.

Steven Tribe
8-Feb-2014, 01:50
So Harold, a possible scenario is that the outer surface of the lens had absorbed moisture during the past 140 years - even though it looked quite perfect? But that warming to 60 degrees was enough to start a chemical reaction/phase change?
There was certainly no water involved during the process.

I note that quite a few of the components of glass are hygroscopic.

Struan Gray
8-Feb-2014, 16:01
I once knew someone who ran an art glass studio that went bust when persistent low-temperature devitrification ruined their productivity. The (porous, impossible to sanitise) lining of the kiln harboured crystalline dust which nucleated devitrification in new pieces at lower temperatures than the process normally occurs.

But even that was a few hundred degrees celcius.

I agree with Harold that you are probably seeing something related to the 'glass sickness' which affects some kinds of vintage glass. It's a combination of migration of metal salts from the bulk to the surface of the glass, and changes in the way dangling bonds in the glass structure are terminated - which can affect the physical structure, making it porous. Had you been using a polar solvent - or an alkaline one - I would have been sure that's what you are seeing, but I'm no expert in what xylene does and does not dissolve when it comes to glass components.

One of the early historical glassworks in Sweden is particularly susceptible to these effects because of the mix of ingredients they put into their melt. There has been quite a bit of work done on finding out exactly what the chemical changes are which take place (mostly potassium migration) and whether there is any cure for sick glass (executive summary: no). Anecdotal evidence from friends in the trade is that healthy-looking glass objects can suddenly develop symptoms if held at elevated temperatures for long periods (hours to days). Sound familiar? There's a useful summary (in Swedish) here:

http://www.raa.se/publicerat/rapp2011_19.pdf

Note that this is indeed related to early attempts to grow a bloom for anti-reflection purposes (or the way glass goes cloudy after many trips through the dishwasher), but it also involves substantial changes to the structure and porosity of the surface layer of glass which make it physically fragile.

Harold_4074
10-Feb-2014, 15:01
Organic chemistry is not my forte, but it is possible that xylene penetration into the damaged glass resulted in some reaction (polymerization, for example) that created stress. It is also possible that absorbed moisture was displaced by the xylene, leading to enough change in stress state to either create or reveal fractures. Your comment that one case involved a hot-air treatement would support this latter conjecture.

The lens with the near-edge damage supports the condensation/corrosion theory; exposed to cyclic temperature changes near the dew point, the metal barrel would cause the edge glass to lag behind rising air temperatures, whereas the main part of the lens could track quickly enough to stay above the dew point. (At certain time of year, the sledgehammer, axes and picks in my garden shed get rusty, whereas the shovels and rakes next to them do not. It took me a while to figure this out...)