Regarding your question of chromium or selenium intensification:
The tests which are shown in the back of A A book "The Negative" , indicate that selenium intensification gives a .40 density increase to a 1.50 beginning density. The intensification continues in a more fairly straight line and seems to exhibit a truly proportional increase to the density of the negative.
The test which is shown for IN-4 intensification (Chromium) show that with a 1.50 beginning density that a .40 density is also obtained. However the curve of this means of intensification indicates tha a fairly pronounced shoulder is inherent. This method gives more early increase in density and does not exhibit truly proportional tendencies.
In summary the curves indicate that when a means of increasing contrast in a negative is desired that selenium is the more effective. Provided of course that sufficient density is present on the negative.