# 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. 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 at the surface 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 substitution of ions creates a compressive force in the sites where it occurs. Since the process is diffusive, the compressive force is produced on the surface of the glass and the interior is put in tension. The compressive force at the surface causes any surface defects, namely scratches, to be in compression. These surface defects being in compression prevents their ability to grow or cause failure of the glass, thus strengthening the glass.

If you make a glass with potassium, you have potassium ions occupying potassium sites, thus no strengthening occurs. Thus the only way to chemically harden the glass is to substitute larger ions into the glass.

Preemptive Question: What about internal defects causing failure?

Gorilla Glass is made via the fusion draw process where molten glass flow over two sides of a platinum trough and meets at the bottom to be rejoined. This process prevents many internal defects to the point that they are very rare and quality control can remove the few pieces with defects that are made.

• 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? May 23 '19 at 16:26
• Or would that actually also make the glass stronger by adding different kind of stress? 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? 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 May 23 '19 at 19:13