If the iron stove-pipe system is adopted, a furnace similar to the one shown at Fig. 10 must be provided, and after an additional few feet of brick flue the iron pipe would commence and turn back upon itself much as the flue in the fire-brick furnace. Proper supports must be provided, and the pipes must be stout and jointed together with expansion joints, otherwise considerable difficulty will be found in keeping a long length of flue pipe perfectly free from leakage. Furnaces on this principle may be designed so that they throw a certain amount of radiant heat direct into the hot-rooms, and they possess this advantage over a mere stove, that they warm the air more gradually. The furnace should be built adjoining the laconicum, the partition wall being of 4½-inch glazed brickwork, having a large number of small openings made therein by leaving void spaces as described further on for the fireclay heating apparatus. Behind this wall the iron flue-pipe should be placed, turning back upon itself, as described above, for perhaps half-a-dozen times, and ending in the vertical brick flue. The furnace itself should be of fire-clay, and so designed that its utmost heating power may be economically employed in warming the incoming air, which should pass over the furnace and iron flues, through the holes in partition wall, and thus into the hot rooms. The flue, if of wrought iron, should be rectangular in section, but if of cast-iron it should be round.

The most economical way of obtaining a high temperature in a small, inexpensive, and unpretentious private bath is by means of a common laundry stove, with a longer or shorter length of iron flue in the apartment. This is the cheapest and quickest method of raising the temperature of a room for sudorific purposes.

Fig. 10.
A Fireclay Heating Apparatus.
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To turn to methods of heating from a radiating surface of firebrick, at Fig. 10 I have given the plan, elevation, and sections of a fireclay heating apparatus. It is constructed wholly of fireclay—fireclay bricks, quarries, and cement. In the main it consists of a long flue of firebricks and slabs, which coils backwards and forwards over itself till the desired amount of radiating surface is gained. Between the coils are spaces for super-heating the air already warmed by passing over the actual furnace and into the warm air chamber, the air passing through by means of perforated bricks. The illustration shows a simple furnace; but it would be an easy matter to improve upon this by providing iron air-tight doors lined with fireclay, for cleansing flues and air-chambers. The example given is only suited to heat a small public bath. For a large set of hot rooms, a compound apparatus could be constructed by placing an additional furnace in a sub-basement, the one on the level of the sudatory supplying radiant heat, and the lower one hot air. Two such apparatus might be placed one behind the other, end to end, or might form the sides of the laconicum; the last plan, however, being the least to be recommended, as in such positions they would not directly radiate their heat into the adjoining hot rooms.

The advantage of such a furnace as that shown is that it supplies radiant heat of a most exhilarating kind, besides a proportion of heated air, and from a fireclay surface, the employment of which renders it absolutely impossible to overheat the air, or to contaminate it by deleterious particles resulting from the decomposition of metal. Moreover, the stoking of this class of furnace requires less arduous attention than an iron stove. Its disadvantage is that, should the temperature of the bath be allowed to fall markedly, it requires some time for the extra heat to be made up again. Inasmuch, however, as fires at public baths must be kept banked up overnight, this is not a matter of importance. It is this very slowness of increase in temperature that constitutes the safeguard against that overheated air, the presence of which we can, with practice, detect by the smell in so many baths. The difficulties involved in the construction of a furnace of this nature relate to the prevention of cracking and consequent escape of sulphurous fumes and carbon into the air. The very simplicity of the construction of the flues and air-chambers constitutes the chief danger, as the chances are that, unless the architect stands by and sees every joint made, the work will be done badly. Absolutely faultless workmanship must be employed throughout, and the fireclay materials must be literally of the very best and soundest description. Every single joint must be perfectly made with fireclay cement or paste. The fireclay bricks, &c., must be selected with regard to the amount of indestructible silica in the clay, consistent with hardness and toughness. Homogeneity of material must be obtained, having regard to expansion and contraction. The same material used for the bricks, &c., worked into a paste, must be employed for the joints.

The design for a furnace on the principle shown at Fig. 10 must be prepared with constant regard to expansion and contraction in heating and cooling. Should this warning be disregarded, fractures will result. It will be seen, upon reference to the plans, that the block of flues and air spaces is left quite free, to allow of any expansion, the connection with the smoke-shaft being by means of an iron flue-pipe, which, being provided in considerable length before passing through the party-wall of laconicum and stokery, by its flexible nature permits any slight movement in a vertical direction. If an "expansion" joint were provided, there would be a sufficient length of iron pipe if it passed direct from the junction with the heating apparatus into the stokery. So much of the iron flue as is in the laconicum must be coated with asbestos or some composition, or the heating will not be wholly by firebrick. The junction of iron flue and heating apparatus is shown by a cast-iron cap sliding over a projecting rim of fireclay, moulded into the last quarry cover, similar to the way in which cast-iron mouthpieces are fitted to retorts.

This heating apparatus is shown visible in the laconicum, but if thought desirable it could be screened by a wall of glazed bricks—9 in. and miss 4½ in. The 4½ by 3 in. holes can be arranged in diamond patterns. This screen wall, however, cuts off a large quantity of radiant heat.

The first flue past the actual furnace—shown with ordinary dead-plate, raking fire-bars, ashpit, fire-door, and ashpit door for regulating draught—has walls 4½ in. thick; above, smaller bricks, 3 in. wide; but in a larger apparatus, 9 in. and 4½ in. respectively would be required. The quarries between flues and air spaces are 24 in. by 24 in. by 3 in., with rebated joints. Larger covers would be more liable to crack at any provocation.

In addition to heating by means of furnaces, steam-heating may be employed, if found, as in many cases it would be, convenient and economical. The chief disadvantage of this method of heating Turkish baths, is the constant danger, however slight, of bursting a pipe in the heating coil, which, by immediately filling the highly-heated atmosphere with vapour, might prove most disastrous to the occupants of the hot rooms, who would be seriously scalded. Nevertheless, the principle has been largely employed in the heating of the most recent Turkish baths in Germany.