Fig. 88.

In the construction of the press, c (Fig. 83) is a block of hard wood, 8 in. square by 5 in. deep, having a hole 6 in. diameter, bored through and highly polished, d is a hard wood board 1/2 in. thick by 11 in. square, having a hole 6 1/2 in. diameter in its centre. The piece of wire netting b, 9 in. square, is clamped between c and d, which are fastened together by several screws. Fig. 84 shows a plan and the position of the screws. 6 is a wooden piston, 5f in. diameter and 1 in. thick, slightly curved on its lower face to correspond with wire g, and on its upper face is a metal plate, 2 in. diameter. Secured on each side of the block c, are two metal uprights f, having a shoulder at the top, on which rests the cross bar g, which is held in place by suitable screw nuts on the extremities of the uprights as shown. The screw shaft h passes through the nut in the cross bar /, and as it is rotated to the right, the piston e is forced downward.

In operating the press, the small nuts holding the cross bar g in place are unscrewed, then the bar and screw are lifted out and also the piston e. The jelly emulsion is then put jn the well a, the piston e is placed on top of it, and after that the screw shaft h and cross bar g. [As the piston e is forced down by the screw shaft, the emulsion is easily and quickly squeezed through the wire cloth.

The plates, prior to coating with the filtered emulsion, are coated with a substratum of:

Filtered water .. .. 30 oz. Potash silicate .. .. 75 m.

The substratum is applied as follows: After the plates are carefully cleaned, they are placed edgewise in a vessel of water, being completely covered by the same. One plate is lifted out at a time, drained a few seconds, and then has poured over it 2 dr. of the substratum solution, so that the entire surface of the plate will be covered, The plate is again drained a few seconds, and treated to a second flowing of the substratum, is again slightly drained, and then set in and dried in the drying box, Fig. 93, which usually takes an hour.

While the emulsion is filtering into the earthen teapot, all the plates are warmed at one time (and they will retain the heat until coated) in the oven of a stove in the kitchen or over a temporary heater. A marble or slate slab, accurately levelled, is provided, of sufficient size to hold ten 6 1/2 x 8 1/2 plates at a time. The marble top of a washstand answers the purpose well. The floor of the dark room is carefully sprinkled, to avoid floating dust during the coating.

Fig. 89 shows the table and utensils used in coating; upon a bracket is the ruby lantern, just below it is a vessel of hot water, in which are immersed glass rods, in front of this is a small box with a strip of wood over the back for holding the silver spoon level, beside the box is the earthen teapot, next to that are two V shaped wood supports for holding the glass coating rods, and near the end of the table are three screw eyes, rising about an inch above the surface. These are sufficiently separated apart to give a good support to the plate, and must be varied to suit the size of the latter. Fig. 90 shows their relative position. The plate, previously coaated with substratum, is now put on the screw eyes and levelled by turning them to the right or left.

Fig. 89.

In coating, use two glass rods and a silver Spoon, the latter to measure with.

Keep these in the vessel of hot water, under the lamp, Fig. 89, when not in use, that they may be clean and warm. Before using, they must be wiped dry with a clean cloth. For coating a 6 1/2 x 8 1/2 plate, use a silver spoon of a size sufficient to hold 4 1/4 dr. of emulsion; for a 5 x 8 plate, 3-dr.

To coat the plate,the stoneware teapot is held in the right hand and the silver spoon in the left; the emulsion is then poured into the spoon until the latter is full, then the teapot is set upon the tablet A glass rod about 1/2 in. diameter is next taken with the right hand, the emulsion in the spoon is then poured upon the centre of the plate in a pool with the left hand (see Fig. 90). The empty spoon is set in the rack box, Fig. 89. With the glass rod placed square across the warm plate, as in Fig. 91, the emulsion is gradually spread out, first by moving the rod to the right, then the left, which carries it to the edge of the plate. Then the rod is slowly drawn toward one end of the plate and then brought back to the centre, thence to. the opposite end, passing through that portion of the emulsion not yet spread.

Fig. 90

Fig. 91.

In case the emulsion has not been brought quite up to the edges of the plate, it can be made to do so, by raising one end of the rod and drawing it along the edges, as shown in Fig. 92. The emulsion will follow the rod, the latter being drawn along from corner to corner.

Fig 92

By* drawing the rod lengthwise to the opposite edge of the plate, and lowering it as. shown in the dotted line, the emulsion may be evenly coated along that edge.

In coating, the glass rod should not touch the plate, nor be too high. Only by practice can one judge the proper distance. Bubbles can be removed, by placing the end of the rod on them, and they will adhere to it; but, one bubble must be removed at a time, and after each removal the rod should be cleaned, or the bubble will be replaced.

Bubbles are • caused by haying the vessel holding the emulsion too for below the mouth, of the. filtering funnel; as the emulsion drops, it forms a bubble. No attempt must be made to. remove a bubble after the emulsion on the plate has set.