Chester F. Stiles
In comparing metol-hydro developers, recommended by the makers and others, one is at once struck with the difference between the various proportions of me-tol to hydrokinone. The metol-hydrokinone combination is an extremely popular one, proven by its universal use for films.
It is a notorious fact that hydrokinone is faulty in its rendering of tones in the negative, especially in under exposures. When the temperature of the solution is low, hydrokinone becomes inert and its developing energy ceases. Metol, on the other hand, is little affected by temperature conditions, and searches out the minute detail of the negative even in the deep shadows. If metol, on the other hand, has any fault it is in its lack of density power, but the hy-drokinone supplies this deficiency and makes, with the metol, a most harmonious developer, which is always under perfect control.
The metol-hydro combination may be worked separately if desired. That is, we may make up a developer of metol and one of hydrokinone; use our metol first to get the image started and follow it up with the hydrokinone portion. By watching the action of the hydrokinone, we shall be convinced that it plays more the part of an intensifier than of a straight developer. This is proven by the fact that it seems to add density to a metol image at a temperature that it would have failed in straight developing without the metol.
In other words, it takes the active metol to attend to the faint impressions of light and overcome the "inertia" of the plates. Just as a bicycle wheel, once set in motion, continues its movement for an appreciable time, so does the hydrokinone pile on its share of the density after being pushed into activity by the metol.
Eminent German scientists have made some very searching investigations on the energy of various me-tol-hydro mixtures. By careful experiment it has been found that a mixture of two parts hydrokinone to one of metol is equal in strength and energy to that of three parts of straight metol. This curious fact suggested the existence of a definite chemical compound of metol and hydrokinone, and experiments prove the conclusion.
Lumiere Bros and Seyewet, Jr., of Lyons, France, announced the discovery of such a compound. Curiously, it explained some troubles which have occasionally been encountered in compounding metol-hydro combinations. The writer has heard several times of the developer chemicals being precipitated and made insoluble when being made up, and it being reasonably certain that cold solutions were not the fault, the trouble was looked for elsewhere.
The writer had several times mixed very concentrated solutions of metol and hydrokinone with similar concentrated solutions of anhydrous soda sulphite. A precipitate formed, and on distribution seemed to dissolve, showing that the precipitate was insoluble in the stronger sulphite but soluble in the weaker. Adurol gave a similar precipitate. The developers which precipitated were those whose metol and hydrokinone presumably were in the exact proportion to form the precipitate, and the strong sulphite of the concentrated developer, of course, prevented its solution.
No attempt was made to explain the precipitation of the concentrated metol-hydro solutions, for the Lumi-•re researches suggest a satisfactory reason;
We would, therefore, recommend the use of metol and hydrokinone in proportions of one to two respectively, and it will be seen that the price of this metol-hydro mixture is but slightly above pyro. Considering the energy of metol-hydro against the pyro, we find the former much the cheaper, and the keeping qualities are such as to enable one to keep strong stock solutions on hand for a long time.
It is desirable, where possible, to use distilled water in making up developers. Developers made with boiled water are also quite satisfactory, as the boiling serves a twofold purpose of expelling the contained air, which oxidizes the developer on one hand, and on the other by preventing the bubbles in the developer from reaching the plate. A number of small bottles filled to the neck will be found more stable in keeping qualities than one large bottle.
Metol-hydro is universally used for developing papers that print by gaslight. The formula below may be used with equal success on plates or films if diluted somewhat.
Metol 60 grains.
Hydroquinine 120 "
Water 32 ounces.
Then add 12 ounces of a sulphite of soda solution testing 60° by hydrometer. Be sure to rinse the white sulphate deposit from the sulphite crystals, if crystals sulphite is used, before the 60° solution is made. Next add 12 ounces of carbonate of soda solution testing 30° by hydrometer, and 240 minims of a ten per cent solution of potassium bromide.
The order of mixing noted above gives a clear solution which is approximately equal to the ready-for-use Velox developers on the market. The quantity of bromide necessary varies according to the water used. Velox prints usually reach their maximum density in about 20 seconds, therefore enough bromide should be added to keep an unexposed sheet from fogging in the developer until 30 or 40 seconds have passed. Perfect balance between the developer and the restrainers gives crisp blacks and avoids the dirty green tones.
Hydrokinone comes in snow white crystals which dissolve easily in water to a colorless solution. We cannot too strongly recommend the use of a pure sulphite in making up developers, especially anhydrous c. p. sulphite. Because of its compact nature, it is easily weighed, and while dry it is practicalty un changeable, besides being more economical on account of the greater strength. If you must use crystal sul phite, rinse it from white impurities of sulphate which are almost always present. Should you weight white crystals in making up solutions, an error is being introduced and the sulphite will come out too weak. A slight rinsing will dissolve the sulphate impurity since sulphate is extremely soluble. Roll the crystal sulphite thus rinsed upon blotters, and then weight it. This method is accurate enough.
Adurol is a modified hydrokinone. It has several good features, but the greatest is its advantage over hydrokinone in not being affected by temperature. Me-tol and adurol make a fine combination, and it gives graduations which are unequalled.