This is a very simple operation, consisting merely in adding to the collodion layer firm and transparent substances, till the cliché attains the proper consistence. To this end, we prepare a normal collodion of the following composition:
| Gun-cotton | 25 | grammes. |
| Sulphuric ether | 12 | litre. |
| Alcohol of 40° | 12 | " |
Lay the proof on a plate of glass, having first turned up the edges all round, so that the liquids to be poured upon it shall not overflow. On the collodion layer containing the image pour the normal collodion, beginning at one of the corners of the proof most remote from the operator. Then incline it slightly, so as to cause the liquid to flow; and, after the entire surface has been covered, the excess of liquid, if any there be, is poured back into the bottle. Then the cliché is laid flat in a roomy box, or in any other place where it will be sheltered from dust,
and left for a few hours to dry. When fully dried, or when it is no longer sticky to the touch, we again, in the same way as before, pour over the layer of normal collodion a layer of caoutchouc dissolved in benzine. When this has become dry, we apply a second layer of normal collodion, then caoutchouc again, and, lastly, a final layer of normal collodion.
