# Can glass be deionized?

Is there a chemical treatment that could remove sodium and calcium ions from the surface of soda-lime glass to turn it into quartz glass, increasing the hardness?

You can indeed "harden" glass by exchanging ions on the surface, but not because it turns into quartz glass.

Quartz glass is mostly so robust because it has an extremely small thermal expansion coefficient, and is therefore mostly free of internal stress due to uneven cooling. Ordinary glass already gets a lot more durable when you temper it, thereby relieving those internal stresses.

Generally, glass breaks under a very small extensional strain. It has a huge $$E$$ modulus and can take a lot of stress, but when you bend it, so the extensional stress and deformation is largest at the surface, where also you always have small scratches etc.), it shatters.

The trick is to put the glass surface under a large compressive stress. That way it can take a lot more pull before the critical strain (deformation) is reached and breakage occurs.

There are basically three ways to do that: Firstly, you can coat the hot glass with another glass that has a lower thermal expansion coefficient. That way the surface gets compressed during cooling. Tricky, and expensive.

Then, the most common way, is to rapidly cool the still nearly molten surface by e.g. blowing cold air on it. The outer shell is already cold, the inner part takes longer to cool and still shrinks, thereby compressing the surface.

The last, chemical trick (making supremely solid glassware) is to exchange sodium ions in the surface layer (100nm or so i think) by potassium ions. This was done in the eighties in the GDR for household glassware, under the brand name "SUPERFEST", by soaking the glasses in molten potassium nitrate (at 400 °C) for twenty minutes or so. The ions are larger, but the silica network lattice does not yield below at least 500°C, so the surface gets compressed by the exchange.

You know you had a glass that was treated in one of these ways when it does not break into large shards, but basically crumbles into small pieces or even dust. The classic example for the latter is Prince Rupert's drop, of the former any car windshield. (It seems the Na/K exchange does not (necessarily) do that, see your mobile phones "Gorilla glas".)

• GDR = East Germany? I haven't heard that term in decades. It reminds me of my dad (in the UK) bringing home a chunk of the Berlin wall from a business trip. And the songs: wind of change, 99 red balloons... Mar 13, 2020 at 0:20
• Chemically hardened glass will not necessarily shatter into dust or small pieces - it depends on the type and degree of treatment. We routinely perform drop-ball testing on K+ hardened glass lenses and they break into typical big chunks when they fail.
– J...
Mar 13, 2020 at 11:54
• @Karl No it doesn't, on human or any other timescale. Mar 13, 2020 at 13:27
• @Karl - Matt's correct. The variation in thickness in old panes of glass if due to the manufacturing process, which inherently produced glass of uneven thickness (see crown glass). With the advent of float glass production in the 20th century, more uniform sheets of glass were produced. The "flowing glass" story is scientifically inaccurate and is a myth. Mar 13, 2020 at 13:49
• @ToddMinehardt I know of course, being a bloody rheologist and wanted to point out exactly that. Total language failure. Tnx.
– Karl
Mar 13, 2020 at 18:01

You could dealkalize the surface of a borosilicate glass to get rid of alkali ions, but I'm not aware of a method that allowed the extraction of alkali metals from the bulk of the glass.

There are probably quite a few ways to modify the surface, but here's a few reported by Yashchishin and Zheplinskii [1]:

Sulfur anhydride or Freon is the most widely used reagent for this purpose. The method consist in that the gaseous reagent is fed into the firing furnace or mixed with the air used for blowing hollow articles (USSR Inventor's Certifi- cate 647273). An interaction between the gaseous reagent and the glass surface occurs in the processes of blowing and firing.

The chemical resistance of glass containers can be increased by means of special treatment by the products of pyrolysis of ammonium sulfate or chloride. This increases the chemical resistance of glass containers by almost a factor of 10. The method consists in that granules of $$(NH_{4})_{2}SO_{4}$$ or $$NH_{4}Cl$$ are introduced inside the items before the beginning of the annealing. In the furnace, the salt decomposes and the resulting gas mixture interacts with the glass, removing alkalis from its surface.

One more note about the hardness: Although fused quartz is thermally more stable than borosilicates, its hardeness on the Mohs' scale is comparable to the latter (5.3 to 6.5 and 6, respectively).

References

1. Yashchishin, I.N., Zheplinskii, T.B. Improving the chemical resistance of glass containers by thermochemical treatment with a reagent solution. Glass Ceram 53, 135–137 (1996). https://doi.org/10.1007/BF01166432