Manufacturer and Builder ?, 1893
A correspondent of the Engineering News says that while experimenting with blue-printing processes with the object of getting bright blues and clear white lines, he found that after the usual washing a bath of quite dilute acid, such as hydrochloric, or, better, oxalic, would often greatly improve the clearness of the prints, a marked cause of dirty blues being a gradual altering of the solutions even when kept separate until just before using, though poor quality of the ammonia citrate of iron seemed to have much to do with the results.
During the experiments he also found that an addition of oxalic acid to the ordinary blue-print mixture materially lessened the time of necessary exposure. The solutions used were:
1. Ammoniacitrate of iron, 120 grains; water, 1 fluid ounce; to which is added a few drops of strong ammonia solution till the odor is quite perceptible.
2. Potassium ferricyanide ,105 grains water, 1 fluid ounce.
3. Saturated solution of oxalic acid. Equal quantities of (1) and (2) are taken (a); and after being mixed (3) is added as required and the mixture used at once.
Taking, say, in the proportion of 10 ounces of the mixture (a) and adding thereto (b) 1 ounce; (c) 2 ounces; or (d) 3 ounces of (3); the relative rapidity of the coated papers will be closely, in very dull light, as 1; 21: 5; 10, (d) paper being thus about 10 times as rapid printing as (a) in the light mentioned. For example, a print was made from a tracing on linen in 35 minutes on February 25, 11.30 A. M. , on (d) paper during a snow storm, the light being quite dull, while ordinary paper takes the greater part of a day in an equal light.
This great difference only holds good in dark, cloudy weather; as, if comparisons are made in direct sunlight, (d) paper is only three to four times as rapid as (a). An explanation of this probably is, that a weak light that will reduce to oxalic acid mixture (partly ferric oxalic) has but a faint starting or continuing action on the ferric citrate, while with a strong light both commence at once.
For all ordinary purposes it is better not to use a greater percentage than 20 per cent. (c) of the oxalic acid solution, as it is difficult to get the lines to wash white with higher percentage, even with thick black lines on the tracing or negative; and the more sensitive the paper the shorter time it will keep good even in the dark, and also the greater care required in its preparation and use.
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