With regard to the spots, it is not easy to know whether they are produced by particles of iron in the paper, or by the oxide of silver. Le Gray says: "If spots should form, produced by the oxide of silver, they may be removed by pouring over the negative some acetic acid, and passing a brush lightly over it."
The second difficulty, want of depth of tone or intensity in the negative, may have been caused by too short an exposure in the camera, or not having used the proper proportion of developing solution. Try the following:
4 oz. dist. water.
8 grains gallic acid.
When this solution has been filtered, add to it ½ drachm of the aceto-nitr. of silver solution, and 1 drachm of acetic acid. I have generally put a little camphor in the gallic acid solution, as recommended by Laborde. It prevents the decomposition of the gallic acid, and renders the image clearer and free from spots. A piece about the size of a pea for four or five ounces of solution.
As to the third difficulty, I believe nothing but replacing the porcelain dishes by glass ones will prevent the dirty marbled appearance in the bottom of the dishes made of porcelain; they are generally rough and uneven on the surface, and there are often what is called "kiln-cracks" in the angular parts. Two months ago I bought two glass dishes; although they are more than double the price of porcelain, I expect the annoyance of dirty dishes is prevented. The glass ones are made quite round at the sides and ends, and of course will be easily cleaned. I am informed they are made in France, but they could be had of English manufacture.
The animal charcoal in the sensitive solution must be shaken up in the aceto-nitrate solution; and when it has become quite clear, the solution before using must be filtered into the dish.
R. Elliott.
Penslur Iron Works.