Bichromate of potash. 16 grammes.
Gelatine. 21/2 ounces.
Water. 20 to 22 "

According to the weather, the amount of water must be varied; but in any
case the solution is a very fluid one. An ounce is about 35 grammes, as
most of our readers know. A practical collotypist sees at a glance the
quality of the prepared plate, without any preliminary testing. A good
preliminary film is a glass that is transparent, yet slightly dull; the
film is so thin, you can scarcely believe it is there. The plate is
slightly warmed upon a slate slab, underneath which is a water bath; it
is then flooded with the above mixture of bichromated gelatine, leaving
only sufficient to make a very thin film. When coated, the plate is
placed in the drying chamber.

_Drying the Sensitive Film_.--Much depends upon the drying. A water
bath with gas burner underneath is used for heating, and a slate slab,
perfectly level, receives the glass plate. The drying chamber is kept at
an even temperature of 50 deg. C.

The object to be attained is a fine grain throughout the surface of the
gelatine, and unless this grain is satisfactory the finished printing
block never will be. If the gelatine film be too thick, then the grain
will be coarse; or, again, if the temperature in drying be too high,
there will be no grain at all. The drying is complete in two or three
hours, and should not take longer.

_The Negative to be Printed from_.--The sensitive film being upon the
surface of a thick glass plate, it is necessary that the cliche or
negative employed should be upon patent plate, or not upon glass at all,