Finally, the process of flattening is apt to introduce defects of its own. The most common of these are scratches arising from marks left by the flattening tool; indeed, in all sheet glass it is quite possible to see, by careful examination of the surfaces, upon which side the flattening tool was used. Sheet-glass thus has one side decidedly brighter and better in surface than the other, the better side being that which rested upon the “lagre” during the flattening process. On the other hand, if the slab itself be not quite perfect, or if any foreign body be allowed to rest upon it, that side of the glass will be marked in a corresponding manner. In the account of the manufacture of sheet-glass given above, we have outlined one typical form of the process, but nearly every stage is subject to modifications according to the practice and particular circumstances of each works. We will now describe one or two special modifications that are of more general importance. First, as regards the melting process, although the tank-furnace has almost entirely superseded the pot furnace for the production of ordinary sheet-glass, there are still some special circumstances under which the pot furnace is capable of holding its own. Thus, where for special purposes it is desired to produce a variety of sheet-glass which, as regards all defects arising out of the glass itself, and especially as regards colour, is required to be as perfect as possible, melting in pots is found advantageous, and for some very special purposes even covered (hooded) pots are used. For such special purposes, too, sulphate of soda is eliminated from the raw materials and carbonate of soda (soda ash) substituted. For the production of tinted glasses also, whether they are tinted throughout their mass, or merely covered with a thin layer of tinted glass (“flashed”), manufacture in pot rather than tank furnaces is generally adopted, the exact nature and composition of the glass being far better under control in the case of pots.