As regards the depth of glass in the tank, practice also varies very much. The advantages claimed for a deep bath are that the fire-clay bottom of the furnace is thereby kept colder and is consequently less attacked, so that this portion of the furnace will last for many years. On the other hand the existence of a great mass of glass at a moderate heat may easily prove the source of contamination arising from crystallisation or “devitrification” occurring there and spreading into the hotter glass above. Also, if for any reason it should become necessary to remove part or all of the contents of the tank, the greater mass of glass in those with deep baths becomes a formidable obstacle. On the whole, however, modern practice appears to favour the use of deeper baths, depths of 2 ft. 6 in. or even 3 ft. being very usual, while depths up to 4 ft. have been used. The question of the proper height of the “crown” or vault of the furnace is of considerable importance to the proper working of the tank. For the purpose of producing the most perfect combustion, it is now contended that a large free flame-space is required. The earlier glass-melting tanks, like the earlier steel furnaces, were built with very low crowns, forcing the flame into contact with the surface of the molten glass, the object being to promote direct heating by immediate contact of flame and glass; the modern tendency, however, is strongly in the direction of higher crowns, leaving the heating of the glass to be accomplished by radiation rather than direct conduction of heat. There can be little doubt that up to a certain point the enlargement of the flame-space tends towards greater cleanliness of working and a certain economy of fuel, but if the height of a furnace crown be excessive there is a decided loss of economy. Flame-spaces as high as 6 ft. from the level of the glass to the highest part of the crown have been used, but the more usual heights range from 2 ft. to 5 ft. The “ports” or apertures by which pre-heated gas and air enter the furnace chamber differ very widely in various furnaces. In some cases the gas and air are allowed to meet in a small combustion chamber just before entering the furnace itself, while in other cases the gas and air enter the furnace by entirely separate openings, only meeting in the furnace chamber. The latter arrangement tends to the formation of a highly reducing flame, which is advantageous for the reduction of salt-cake, but is by no means economical as regards fuel consumption. On the other hand, by producing a perfect mixing of the entering gas and air in suitable proportions, the other type of ports can be made to give almost any kind of flame desired, although their tendency is to form a more oxidising atmosphere within the furnace. The latter type of ports, although widely varied in detail, are now almost universally adopted in sheet tank furnaces.