It is not necessary to heat the kiln each time before firing, as moisture is not expelled from fire-brick into the oven as it is from an iron firepot. Allow the oil to flow in a small or broken stream for about 15 minutes, and then increase a little. _The main point is to feed the oil very slowly at first until the burner_ has become hot, when it may be increased gradually. The first red will show in about 40 minutes. From this time on, the oil may be increased as fast as the burner will consume it without overflowing. After the interior of the kiln has been thoroughly red for some time it will grow paler in tone and a glow commonly called Sunshine will spread over it. This is the point to turn off the oil, and stop the fire. You can see whether the gold has changed color, and whether the pieces are glazed, especially any dusted tints. If your colors come out dull, and if your rose or carmines are brick color instead of a clear pink, or if your gold rubs off, you may know that you have not fired long enough. If your pinks or carmines have turned purple, you have overfired. A few trials will enable you to know for yourself, just the right length of time.
It takes from one hour and fifteen to an hour and thirty to forty-five minutes. The time varies, depending upon the draught and the way the oil has been fed.
The mistake is too often made, of firing by time entirely, instead of being guided by the appearance of the kiln. After having fired several times, one will have ascertained pretty nearly just how long their kiln takes to fire, and this time is not apt to vary greatly. Yet on some days, depending upon different conditions, it may take a trifle longer or less time, and so let your reason, rather than the clock, dictate the proper firing.
The hole under the door is intended for a peep hole, to see the condition of the fire in the combustion chamber, and should be left closed. When the chimney draft is not good a carbon may form over the burner in this combustion chamber, and by the use of a small poker through this hole it may be removed and taken out through the burner without stopping the firing.
If the kiln does not seem to respond properly, and in a reasonable length of time, you will, doubtless, find that there in insufficient draught in the chimney. This is a most important consideration, and if there is any cause, such as a stove, or fire-place connected with the same flue which cuts the draught, it must be removed. If attached to the same flue as a furnace or stove, the latter must be entirely cut off _when the fires are out in the summer_.