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Perhaps I shouldn't have said the "2% rule". The article by Lyle et al specifically says that "alumina exhibits the greatest improvement in chemical durability when the amount is 1/8th of the alkali content of the glass" As most studio glasses have alkali levels around 16-18% then 1/8th equates roughly to 2%.
The article does go on to make an important observation that a glass of 17% soda and 1.5% alumina has the same resistance to chemical attack as a glass with 16% soda and 0.5% alumina.
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