One soon learns to pack a biscuit kiln, using saddles, spurs, stilts, thimbles, bits of tile or biscuit, and sand or flint as necessary. The thing that is a little difficult to realize at first is that built or thrown shapes, and still less tiles or modelled work, should not be hurried. Twelve hours is none too long to give to a 5⁄8” tile in the biscuit kiln. Although to all appearances thoroughly dry, the least hurry generates steam which will ruthlessly blow our best effects to bits. In packing, two cones or temperature indicators (Fig. 46) are placed somewhere near the middle in a position easily seen during the firing from the spy hole.
These cones are made of different compositions which melt at varying temperatures. Thus if the firing point of a body is known, a cone of that degree is used and the firing continued until the cone bends. This it does soon after it assumes the colour of the surrounding muffle.
To eliminate the uncertainty that is likely to be present at the first few firings it is as well to use two or even three cones, one just above and one below the correct temperature. Placed in order there is little chance then of over- or under-firing unless so
much sulphur gets into the kiln that the cones harden and refuse to turn. Calorites are sometimes used but are not so reliable. The cones may be sloped to insure bending to right or left, as a cone bending towards the spy is deceptive.
Fig. 46
A trial piece of biscuit being placed near the spy hole, the next thing is to close the muffle. In a kiln with hinged doors the spy hole is fixed and this fact has to be taken into account. But with a bricked-up door the spy and vent may be left where it is most convenient. These orifices have plugs that fit them loosely so that if necessary they can be pulled without disturbing the clamming. This clamming or stopping is a mixture of sand, sieved dust, ground pitchers, or other infusible siftings held
together with a very little waste glaze and water. Where much is required, moist sand will suffice. This is plastered into the cracks that would otherwise let heat out of, or air into, the muffle, and so seals up the door. With hinged doors little stopping is required, except round the spy or vent. The crack above the door should not be clammed until the muffle begins to get warm. With this done and the two plugs out, all is now ready to light up.
With an English kiln, a good middle course is as follows: For cone .01, taking 12-14 hours. See that all the burner taps are off with the main cock on one third to one half. Take the reading of the metre. Pull the air regulators right back and the dampers out nearly half. Then take out the plug of the lighting hole and insert a taper. Turn on tap number 1. When lit, withdraw the taper and turn on tap number 2. Turn down to about one half and continue until every burner is lit, making sure that each one is burning freely with a yellow flame. With a kiln having twelve burners turn out all but numbers 3, 6, and 9. With these on one third, very gently push forward the air regulators until a roaring noise tells that air is being admitted to the bunsen burners.
The flame at this time should be blue, and the stopper should be replaced. If the flame appears at all fierce, turn the taps down a little. If turned