The actual tanks or basins which contain the molten glass in tank furnaces are also built of large blocks of fire-clay, but these are made of the best procurable materials, and should receive at least as much care in every respect as crucibles; it is true that their shape and size gives them greater strength, but on the other hand these blocks are expected to resist the contact of molten glass for very much longer periods of time than the average crucible. To understand the requirements for tank-blocks it is necessary to anticipate the next section to the extent of stating that in tank furnaces the glass is contained, during melting, refining and working, in a basin built up of large blocks. These blocks are not cemented together in any way, but are built up “dry” and are supported on the outside by a system of iron bars and rods. The molten glass penetrates between the blocks to a certain extent, but as the outside of all such blocks is intentionally kept as cold as possible the glass rapidly stiffens as it penetrates further into these interstices, and this stiffened glass effectually binds the blocks together and prevents all leakage. It will thus be seen that the blocks are exposed to the full heat of the furnace and to the corroding action of the glass on the inner side, but are kept cold on the outer side. As this state of affairs tends to produce cracks, these blocks are necessarily made of rather coarse material. On the other hand, the material of a block never gets so hot as the wall of a crucible, which is heated from both sides, so that extreme refractoriness is not so essential. It is impossible, within the limits of this chapter, to go into the details of the choice of materials for tank-blocks; it is a subject upon which no finally satisfactory conclusion has yet been reached, and what has been said above will suffice to show the nature of the considerations upon which such choice must be based.