# Why isn't 'chemically-strengthened glass' made with potassium carbonate to begin with?

Instead of making potassium-strengthened glass by creating ordinary soda-lime glass first, then replacing the sodium atoms/ions with potassium by putting the glass in a bath/solution of potassium nitrate, why not replace the 'soda' (sodium carbonate) in the initial process(es) with potassium carbonate?

• – A.K. May 20 '19 at 21:35
• @A.K. That question (and your answer to it) is actually much more interesting than this one. – Dmitry Grigoryev May 21 '19 at 11:52

The potassium is not added first because the potassium does not intrinsically make stronger glass, it is the substitution of a larger ion for a smaller one that does.

To understand why ion exchange strengthens glass, you have to understand why the ion exchange makes the glass harder. The process of ion exchange hardening is done at a temperature that allows ion diffusion, but disallows a reconfiguration of the glass structure or relaxation of bonds (i.e. below $$T_{g}$$, the glass transition temperature). When sodium atoms are replaced by potassium in glass, the potassium ions occupy a site that is sized for sodium; this mismatch creates the compression that in turn gives the glass its scratch resistance. Thus the only way to chemically harden the glass is to substitute larger ions into the glass.

If you make a glass with potassium, you have potassium ions occupying potassium sites, thus no strengthening occurs.

• Interesting, is there a way you could make softer glass by replacing potassium with sodium instead? I'm not sure what use that would have, but is it chemically possible? – Kevin Wells May 23 '19 at 16:26
• Or would that actually also make the glass stronger by adding different kind of stress? – Kevin Wells May 23 '19 at 16:27
• @KevinWells It would not. – A.K. May 23 '19 at 16:46
• Do you mean it wouldn't make it stronger or that it would not be possible? – Kevin Wells May 23 '19 at 16:46
• @KevinWells potassium does not cause stress by being in the glass as any stresses present will relax above the annealing (or melting) temperature. It is only by ion substitution that strengthening will occur. – A.K. May 23 '19 at 16:50

Chemically-strengthened glass is similar tempered glass in that the outside of the glass is under compression, while the inside is not compressed. If all the sodium in chemically-strengthened glass were to be replaced by potassium, there would be no difference in stress between the interior and the exterior layer.

How does this difference between layers help make the glass stronger? As in prestressed concrete, this makes the substance stronger under tension because the initial stress counteracts that force.

As A.K. notes, it is the substitution of larger atoms for smaller atoms that adds stress to the glass, making it stronger. So you can't increase the stress by starting with a larger metal, but you could on the other hand start with a smaller metal:

Chemically strengthened lithium aluminosilicate glass (Int. Patent Application)

• All credit is due to A.K., I just made some minor edits – llama May 23 '19 at 19:13