Ferrous oxalate works quite well with plates of that kind; so do soda
and potassa developers.

For soda developers, Eder uses a solution of 10 parts of pure
crystallized soda in 100 parts of water. For use, 100 c.c. of this
solution are mixed with 6 c.c. of a pyrogallic solution of 1:10, without
the addition of any bromide.

More pleasant to work with is Dr. Stolze's potassa developer. No. 1:
Water, 200 c.c.; chem. pure potassium carbonate, 90 gr.; sodium
sulphite, 25 gr. No. 2: Water 100 c.c.; citric, 1½ gr.; sodium sulphite,
25 gr.; pyrogallol., 12 gr. Solution No. 2 is for its better keeping
qualities preferable to Dr. Stolze's solution.[A] The solutions when in
well stoppered bottles keep well for some time. To develop, mix 100 c.c.
of water with 40 min. of No. 1 and 50 min. of No. 2. The picture appears
quickly and more vigorously than with iron oxalate. If it is desirable
to decrease the density of the negatives, double the quantity of water.
The negatives have a greenish brown to olive-green tone. A very fine
grayish-black can be obtained by using a strong alum bath between
developing and fixing. The same bath after fixing does not act as
effectual in producing the desired tone. A bath of equal volumes of
saturated solutions of alum and ferrous sulphate gives the negative a
deep olive-brown color and an extraordinary intensity, which excludes
all possible necessities of an after intensification.

[Footnote A: 100 c.c. water; 10 c.c. alcohol; 10 gr. pyrogallol; 1 gr.
salicylic acid.]

The sensitiveness with this developer is at least equal to that when
iron developer is used, frequently even greater.