For these reasons in most works crucibles have been abandoned in favour of muffles, a large number of which are built in one furnace. The charge is placed in the muffles; after completion of the calcining it is quickly raked out, and the still hot muffle at once recharged, thus the losses of time and heat are reduced to a minimum. The muffles are generally about 1 metre long, 1 metre wide and 30 to 40 centimetres high. They are made of fire-clay, and, when kept in uninterrupted use, last a long time. The muffles have a small opening at the back which communicates with the furnace, so that the gases evolved have a free outlet without reaching the working place. The front of the muffle is closed by an iron plate in which are small openings, through which the contents can be examined without the entry of much air. Three muffles are generally built in a furnace, but, by suitable alterations in the construction, a considerably larger number may be heated in one furnace. Whatever the number of muffles, the furnace must be so arranged that the fire can be controlled at will. This is best done by a good damper; all the muffles must be uniformly heated. J. Curtius recommends cast-iron cylinders, lined with a thin layer of fire-clay. For this purpose a covering of fire-proof cement is used to protect the retorts. This covering, as it is gradually destroyed, may be renewed by smearing over the damaged places with fire-proof cement, mixed with water or some binding material. Aluminium silicate, graphite and coke may also be used. The retorts, _a_ (Fig. 28), project through the wall of the furnace at one end. They rest on fire-proof stone bridges, and can be put into communication with the air by the pipe, _m_, which can be closed air-tight. Short pipes, _k_ and _d_, which can be closed, connect the interior of the retort with the cooling and collecting chambers, _P_ and _g_, the latter of which is connected by _r_ with the vessel, _h_, where the gaseous products are absorbed. The porous plate, _f_, prevents the charge in the hinder part of the retort from becoming open and falling to pieces during stoking. In order to obtain a regular heating of the retorts the fire gases passing through the space, _q_, are led away at two opposite places by flues into the main flue, _F_. The intimate mixture of finely-powdered materials is introduced into the hinder part of the retort, the porous plate, _f_, placed in position, the cover, _t_, closed up air-tight, and the connection with the chamber, _P_, closed. On heating, the volatile products pass through the porous plate and the pipe, _d_, into the chamber, _g_, the more volatile portions, without mixing with the furnace gases, passing direct to the absorption vessel, _h_, or to a lead chamber, whilst the sulphur which distils over remains in _g_. When the reaction is finished, by opening the cover, _t_, and the tube, _m_, the crude green ultramarine in the retort can be rapidly oxidised