Many other modifications are commonly practised locally, their object being for the most part to yield bread which suits the local taste. It will suffice to mention one which has a special interest. In this method the essentially interesting point is the preparation of what is known as a ferment. For this purpose a quantity of potatoes is taken, about a stone to the sack of flour. After peeling and cleaning they are boiled and mashed up with water into a cream. To this a small quantity of yeast is added and the mixture kept warm until fermentation ceases, as shown by the cessation of the production of gas. During this fermentation the yeast increases enormously, so that a very small quantity of yeast suffices to make enough ferment for a sack of flour. The flour is now measured out into the trough, and the ferment and some additional water and salt added so that the whole can be worked up into dough. Scaling, moulding, and baking are then conducted as before. This method was in general use years ago when yeast was dear. It has fallen somewhat into disuse in these days of cheap compressed yeast, in fact the use of potatoes nowadays would make the process expensive. In private houses and in the smaller local bakeries the whole of the processes described above are carried out by hand. During the last few decades many very large companies have been formed to take up the production of bread on the large scale. This has caused almost a revolution of the methods of manipulating flour and dough, and in many cases nowadays almost every process in the bakery is carried out by machinery. In many of the larger bakeries doughing and kneading are carried out by machines, and this applies also to the processes of scaling and moulding. A similar change has taken place too in the construction of ovens. Years ago an oven consisted of a cavity in a large block of masonry. Wood was burned in the cavity until the walls attained a sufficiently high temperature. The remains of the fuel were then raked out and the bread put in and baked by radiation from the hot walls. Nowadays it is not customary to burn fuel in the oven itself, nor is the fuel always wood or even coal. The fuel is burned in a furnace underneath the oven, and coal or gas is generally used. Sometimes however the source of heat is electricity. In all cases it is still recognised that the heat should be radiated from massive solid walls maintained at a high temperature. In the latest type of oven the heat is conducted through the walls by closed iron tubes containing water, which of course at the high temperatures employed becomes superheated steam. It is recognised that the ovens commonly provided in modern private houses, whether heated by the fire of the kitchen range, or by gas, are not capable of baking bread of the best quality, because their walls do not radiate heat to the same degree as the massive walls of a proper bake oven.