The muffles used for finishing the ultramarine are similar to those used in making green ultramarine; they are generally 50 centimetres wide and 100 to 120 centimetres long. A muffle of this size will contain 6 to 18 kilogrammes of green ultramarine. The temperature is gradually raised to a gentle red heat, each muffle being provided in front with an iron plate which prevents the cooling of the contents by the external air. When the charge in the muffles is carefully observed, it is noticed that the blue colour first appears at the surface and the edges, _i.e._, at those places where the oxygen of the air has unrestricted access. The mass is frequently turned over with an iron rake, in order to expose all parts to the action of the air; samples are frequently taken in order to observe the moment at which the colour has reached its greatest intensity, when the heating is at once stopped. By proper treatment about half an hour is required to convert the charge of a muffle of the above dimensions into ultramarine of the deepest blue. The hot mass is drawn out of the muffles and spread out upon flags so that it may cool quickly. During the cooling the colour is often observed to become considerably darker, which is a sign that the ultramarine has not been heated for a sufficient length of time. Mechanical arrangements are often used to continually turn over the ultramarine in the muffles, which are then made in the form of cylinders in which the ultramarine is continually turned by a stirrer provided with wings. The whole arrangement is very similar to the apparatus used for roasting coffee. The ultramarine is then carefully ground to a very fine meal; it is tolerably hard, and granite stones should be used, ordinary stones would be too quickly worn down. It is not possible, by the most careful grinding, to convert ultramarine into that condition of fine division which is necessary if it is to be ground into paint; grinding must be followed by levigation. The various qualities and shades brought into the market differ only in regard to the fineness of their particles; they do not vary in chemical nature, for the substance has always the same composition. It occasionally happens in ultramarine works that a charge does not turn out a bright blue; the product is then sold as inferior quality at a lower price, or is mixed with a larger quantity of good material. In working by a settled process the different shades of ultramarine vary in composition between narrow limits; the difference is caused by variations in the raw materials.