ARRANGEMENT OF FORMULAS
To facilitate their intelligent use and application, all formulas are offered
not only in a uniform typographical arrangement, but also in a uniform standard
of volume, whenever possible. The standard adopted is the metric system, found
most practical and useful for many reasons: formulas uniformly expressed in
these units are readily understood, easily compared with one another, readily
converted into larger or smaller volumes by simple multiplication or division.
Especially when a formula is expressed in the metric system, it immediately
creates a mental picture of the relative concentrations of the various
ingredients, as it gives visually a percentual relationship of each ingredient
to the total volume.
MAKING SOLUTIONS ACCURATELY
As a matter of good chemical practice, applying equally well to photography
as to other chemical sciences, formulas were standardized as to their relative
strength; almost every formula is concluded by the expression: "Add
cold water to make 1000ml." This
recommendation is based upon the fact that when the chemical is dissolved in
water, the volume of the solution is usually
greater than that of the water, because the
particles of the chemical occupy a definite space in the solution. For this
reason, it is always best to dissolve the chemicals in a volume of warm
water substantially less than the final
volume of the completed formula and then add cold
water to make up the stated volume. This also makes it easier to dissolve
the chemicals, and results in greater uniformity of the concentration in its
final solution. It is obvious that if the preparation of the solution were
started with the amount of water stated for
the final volume, the completed solution would be of weaker concentration,
though of greater volume, than originally specified. In many instances when
the solution is being prepared with warm water
(about 125°F/52°C), frequently cold
water or cracked ice can be added, bringing the solution to the desired
temperature more rapidly. The practice of dissolving chemicals in warm
water and the subsequent addition of cold
water or ice to make the specified volume
definitely results in consistently uniform solutions of standard useful
properties.
The term "stock solution" is applied throughout to every formula which needs to be diluted with water for use. Stock solutions of formulas frequently eased are most economical and practical. They keep well for long times if stored in well-filled and thoroughly-stoppered bottles, preferably of amber glass. Bottles containing stock solutions should be kept in cool, dark places and suitably labeled. As quantities of stock solutions are removed for site, it is a good practice, in order to keep the level of the remaining portion of stock solution as close as possible to the closure, to fill the bottle with glass marbles. Working solutions, that is diluted solutions, should be discarded after use. When a dilution of a stock solution of any chemical is called for, such as "dilute 1:3," it is implied that the first number refers to the quantity of the concentrate (stock solution), and the second number refers to the desired quantity of water. Therefore, in the above example, you would be dealing with one part of the concentrated solution, a developer, for example-and three parts of water, to make a working solution of a total of four parts.
COMPOUNDING YOUR OWN vs. PREPARED
The chief aim of presenting a photographic formula in its
true version is to acquaint photographers with its exact structure. Frequent
reference to authentic formulas invariably results in better understanding of
their workings and performance. Of course, a photographer should be able to
prepare the formula themselves, with full confidence that he is following the
best recommended practice to the letter, providing he is weighing and measuring
everything correctly and that he is using pure, dependable ingredients. Also,
providing that he is carefully following directions for the preparation and use
of the formula. Most manufacturers of photosensitive materials offer the very
formulas they publish conveniently put up in powder or liquid form, packed in
tins, cartons or bottles, requiring merely that their contents be dissolved in
a stated volume of water to be available for
instant use. These prepared formulas are available in a variety of popularity
used quantities and having many advantages. Being prepared by the manufacturer
of the photosensitive material, they are dependably accurate with a known
content. Their use eliminates waste. In this form they can be stored almost
indefinitely and be easily transported. Their cost is low and their use most
economical. One needs no scales when preparing solutions from them: a graduate
and a stirring rod make them immediately available for use. One need not worry
about inadvertently omitting one of the ingredients, or putting another
ingredient into the solution twice. Inaccuracies in weighing and measuring are
also eliminated. The cost of such prepared formulas is low; while slightly
more expensive than formulas compounded from individual ingredients, the slight
additional cost is definitely offset by the greater convenience and saving of
time. Consistent use of prepared formulas makes it unnecessary to maintain
stocks of various separate ingredients in a variety of containers, which is
frequently coupled with possible waste from deterioration or pollution.