Hard and soft glass

[Buttercup]

Is there a concise definition or way of identifying which glass is 'soft' and which is 'hard' art glass, please? I often see the expressions but don't
really know what they mean other than that they fuse and melt at different temperatures. I feel I should know this but hope someone can educate me! Thanks, Jen

[Morganica]

Actually, it is an industry term that we don't really use correctly. "Hard glass" is borosilicate glass, and all fusing and casting glasses are considered soft. Hard glass has a low COE and takes more heat work before it begins to move, so it is the choice in, say, scientific glassblowing because it can be rapidly heated and cooled without cracking. If you're a lampworker, there are huge differences in the way hard and soft glasses are worked, and even in the equipment required to work them.

In kilnwork, though, youre not really interested in one glass by itself, but in how it behaves with another glass. Even though it's not really correct to call soda lime "hard," it's a way to compare. You can also use " stiff." Pale, opaque soda lime glass tends to be harder than darker soda lime transparents. Lead crystal is softer than soda lime.

I'm sure the manufacturers have precise tables showing the relative softness of their glasses, but I just tend to follow the opaque-light/transparent-dark rule. I know that if I'm mixing equal amounts of white and black in a piece there is a very slow schedule in my future.

[Buttercup]

Thank you so much Cynthia. That's a concise clarification. It will be much more accurate than the 'bite' test.
Much appreciated. Jen