Each chamber, if made about 36 feet long, 15 feet mean width, and 8 feet high, will hold 25,000 bricks.
12 x 25,000 = 300,000 bricks may therefore be burnt in the whole kiln every twelve days, or (as the bricks are not filled in or unloaded on Sundays) say once a fortnight.
Such a kiln will therefore burn some four or five million bricks per annum.
The great drawback to the use of Hoffmann's kiln is the first cost of its construction.
It is necessary to burn some six to ten million bricks before the saving in fuel has compensated for the cost of building the kiln.
It is therefore not adapted for burning bricks for special works unless they are on a very large scale.
It is, however, the most economical form of kiln for permanent brick-making works turning out a large annual supply of bricks.
The advantages of Hoffmann's kiln are -
1. Economy of Fuel. - In Scotch and similar kilns a great deal of the heat from the burning fuel, and also all the heat from the bricks when cooling, passes away and is wasted; by this kiln they are both preserved, and utilised in drying and heating the bricks before burning.
The result is that only 2 to 3 cwt. of coal dust and slack, costing 4d. 01 5d., are required per 1000 bricks, instead of half a ton of good coal, costing 4s. or 5s.
The prices given are those quoted by the patentees, and vary of course in different localities.
2. There being no rapid draught, the hot gases fill the chambers, and the bricks in all parts of the kiln are burnt equally well.
3. The bricks can at any time be examined, and the burning regulated, through the fire-holes.
4. As the fuel is thrown into the chambers after they are at a high heat, wood, turf, or coal can be used.
5. The charging and emptying of the kiln goes on continuously and without interruption, so that a regular supply of bricks can be maintained.
6. The height of the chambers, only 8 or 9 feet, is such that there is no danger of crushing the lower courses when the bricks are raw or at a high temperature.
7. The bricks are not liable to injury by sudden changes of temperature.
8. There is no smoke, as the combustion of the fuel is perfect. Modifications of Hoffmann's Kilns are used in different parts of the country.
In many of them the chambers are differently arranged. They are often placed in a straight line, and the waste heat from each is utilised in a somewhat similar manner. Among these may be mentioned Lancaster's, Morand's. Clayton's, Pollock and Mitchell's, and Chamberlain and Wedekind's kilns.
Bull's patent Semi-continuous Kiln is said to utilise the waste heat and thoroughly to consume the fuel, without expense in construction of a very large kiln. The expenditure of coal is stated to be about 3 cwts. per 1000 bricks.
The bricks are packed in a somewhat elaborate manner. The whole construction is fully explained in Engineering of the 22d October 1875.
Cupolas, or, as they are locally called, ovens, are small circular domed kilns. They are used in Staffordshire for the celebrated bricks of that district (see p. 108). They are sometimes used in other localities, and also for burning firebricks.
An immense number of different kilns are in use for burning bricks and tiles of special descriptions. New forms of kiln are invented nearly every week. It would be impossible, for want of space, to describe even a few of these, and such a description, if given, would be interesting to the manufacturer rather than to the engineer or builder.
Classification of Bricks - Building bricks may, for the purposes of the engineer or architect, be divided into three classes.
The best of these are selected for fronts, and are termed facing bricks.
Specially hard varieties are used for coping, also for paving, quoins, and other positions where they will be subjected to unusual wear.
3. Uhderburnt and misshapen Bricks, only fit for inside work. Of each of these classes there are in most brickfields several varieties, varying in quality according to circumstances. Their general characteristics are, however, as follow: -
Cutters or Rubbers are purposely made sufficiently soft to be cut approximately to the shape required with a trowel, and then rubbed to a smooth face and to an accurate shape.
To ensure this they are made of washed earth carefully freed from lumps of all kinds, and uniform in composition throughout its mass.
The best rubbers are burnt to a point a little short of vitrification.
Inferior kinds are often stinted in firing; the cohesion between the particles is small, and they are easily destroyed by rain or frost.
For the sake of durability it is better to avoid rubbers in all exposed work, and to use "purpose-made" bricks moulded to the shape required and thoroughly well burnt.
This is often done in good work.
The characteristics of good rubbers are mentioned at page 111.