Figure 20, the copper tank, the outside dimensions of which are 37 by 31 inches, the flange fitting on top of the jacket; the depth of the tank in the centre is 3 inches; the inlet pipe is 1 inch bore, and 6 inches long, the bend being taken high enough to be about 2 inches above the top of the tank.
The base is placed upon the table, then the iron jacket on the top of the base; then the copper tank is fitted into the top of the jacket; upon the top of the tank is placed a slab of marble or slate 37 1⁄2 by 31 1⁄2 inches, cemented by means of plaster-of-Paris. The wooden top of the oven is fitted into position, the canvas being nailed on last. But, before doing this, it will be as well to fix the bearers as at C C C, Fig. 21, with corresponding ones on the other side, so that slats may be placed across for the purpose of holding heliotype plates, and for drying transfer paper, etc. The interstices between the edges of the slab and woodwork should be made good, either by means of wooden laths or cement.
Fig. 21.
The oven being erected, the first thing to do will be to fill the copper tank with water; then level the slab by means of the screws, or if the bottom ends of the base are shod with sheet iron, wedges may be used to level with. The drying oven must be kept in a room at a uniform temperature of about 60° F.
In using the oven after the plates are dry (after rinsing), place a piece of clean, dry, brown paper upon the slab, then place on this paper four plates 16 by 13 inches—which are the size and quantity for which the dimensions of the oven have been calculated; then place a thermometer between the plates, and close the oven. After the water has been brought to boil, the plates will be ready for coating; but, if the thermometer registers over 120°, turn off the gas, coat the plates, again close the door, and let the plates dry.
One of the first things the student must do is to familiarize himself with the oven. Note the time taken to bring the water to boil; the temperature of the {170} slab, and the time taken for the temperature to fall after turning off the gas. A day spent at this will teach far more than it is possible to do by writing.
Do not try to improve the oven by having the door to open any other way than by sliding. If made to open in or out, a draught of air is created which will cause mischief. Petroleum, instead of gas, may be used for heating the water.
Instead of using glass plates for carrying the collotype film, copper or brass plates may be used. They should be grained with fine sifted graining sand and a stone muller, then washed with plenty of fresh water, and coated with
| Stale Beer | 5 | ounces. |
| Water | 5 | ounces. |
| Silicate of Soda (syrup) | 2 | ounces. |
| Tannin | 1 | grain. |