I read a post somewhere recommending a certain way to remove some kind of residue, but the post said it would only work for (at least) "semi-vitreous surfaces".
So, glass would fall under this category. But what else? What kind of surfaces/materials do people mean when they use this term?
When specifying porcelain for tiles or work surfaces, one can have, in ascending order of porosity, non-vitreous, which holds the most water, semi-vitreous (in the middle), and impervious, which holds little or no water. In view of the stain-removal context, I would interpret "at least semi-vitreous" to mean "semi vitreous or impervious".
As you say Vitreous means glass. A fusion of Alumina amd Silica with a metallic Oxide which affects the temperature of the melting point and the colour.
Glazed tiles are fired at a temperature which fuses the body of the tile (mostly alumina) with silicates and white, clear or coloured oxides.
The same fusion produces a glassy surface on china and pottery.
Enameled jugs and bowls are also covered in aluminium-silicates (English spelling here).
So, Michael Harvey has given you the technical meaning, I hope this gives the in
formation to interpret the instructions.
