Comment by mannykannot | Hacker News Reader

Comment by mannykannot | Hacker News Reader

This article's brief explanation of how the process works seems somewhat misleading, as it claims that the ion substitution creates tension in the glass, which makes it harder for cracks to spread - but cracks propagate under tension.

As I understand it, the substitution of larger potassium ions for smaller sodium ions in the surface layer creates a compressive stress in the surface, and while this results in a counterbalancing tension in the interior, it is the surface compression which inhibits crack propagation.

This is how tempered glass works, except the surface compression and interior tension is the result in the greater shrinkage of the slowly-cooling interior. The interior tension is also the reason why these glasses shatter into tiny pieces when they do break (see also Prince Rupert's drops: there are many videos on Youtube.)

I am wondering if the chemical process is slow, which might be another reason for it not being adopted for ordinary objects such as drinking glasses.

It's the same process used on modern phone screens (Gorilla Glass). Chemical strengthening.

https://en.m.wikipedia.org/wiki/Chemically_strengthened_glas...

And yes, it works the same way as thermally toughened glass (inducing a compressive stress near the surface).

I see this article says that chemically strengthened glass can be cut, and while it becomes weakened in the vicinity of the cut, it does not shatter as a result. The Wikipedia article does not explain in any detail why this is so, but it suggests that there is less tensile stress than in tempered glass. Perhaps this is because the latter needs the internal tension to create the surface compression, while in chemically toughened glass, the compression is generated within the surface layer?

I have seen it said about Supafest drinking glasses that they do shatter into tiny pieces when they do break, though I do not know whether the source was reliable. If this is the case, then it suggests there is some difference between the Supafest and Corning / Gorilla Glass processes.

It's due to the thickness of the compressed layer at the surface. It's often thinner (but more intense therefore harder) in Gorilla glass, so I suppose there isn't enough tension in the interior to make it shatter.

There's always a corresponding tension in the interior, the sum of forces in each direction are balanced. But if the outer compressed region is thin, you won't need as much internal tension.

> This article's brief explanation of how the process works seems somewhat misleading, as it claims that the ion substitution creates tension in the glass, which makes it harder for cracks to spread - but cracks propagate under tension.

The article uses “tension” informally, meaning under mechanical stress. I would not read too much into that. But yes, you are right.

> I am wondering if the chemical process is slow, which might be another reason for it not being adopted for ordinary objects such as drinking glasses.

The fact that such glasses tend to fail catastrophically and shatter in small shards is a problem, for something that is used in homes, often with children, and that is bound to break at some point. Not great from a safety perspective.

The cleanup after a broken tempered glass breaks isn't worse than after a regular glass in my experience. There are more shards, yes, but they are individually less dangerous. And in either case the annoying thing is having to be thorough. Regular glass shards seem to be better at hiding.

A random tip for the thread: when looking for glass or other small objects, shine a bright flashlight parallel to the floor: all detritus is made visible by casting longer, harder shadows.

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