Plate Backing 
CARAMEL BacKING.—A non-actinic coating is applied to 
the back of glass plates to prevent halation, or the reflection 
of brightly lighted objects from the back surface of the glass. 
To be effective the backing must be in optical contact with 
the glass, and should theoretically be of the same refractive 
index. 
Caramel or burnt sugar does not dry, but a crystal caramel 
powder, which dries hard, is obtainable commercially. This 
can be made as follows: place some white sugar in a stewpan 
and heat over the gas, stirring continuously until it melts. 
The temperature will then rise very rapidly to 222° C. 
(430° F.). Continue heating for 15 minutes, stirring all the 
time; bubbles of gas will be given off and a clear red fluid 
formed, which gradually thickens. The heat should not be 
continued more than 5 minutes longer at this stage, or the 
product will be spoiled. Then pour out on a sheet of metal 
or slate, and it should set hard and brittle. Break up into 
small pieces, place in a wide-mouthed bottle, add water equal 
to one third the height of the fragments, and leave for three 
days. Add ten times the volume of wood or denatured alcohol 
or methylated spirit, and shake well two or three times in 24 
hours. Pour off the alcohol, which extracts the sticky part, 
and add sufficient water to the residue to make a fairly thick 
liquid. This dries hard but brittle. If one eighth of the 
original mixture, before the addition of the alcohol, be added, 
it loses its brittleness. Instead of dissolving in water, it may 
be poured out in a flat dish, allowed to dry, and then 
powdered. A suitable backing mixture is: 
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