Hello. I was in the net and I find this:
What do you think? I never heard to boil emulsions before, but the color obteined is familiar.
Boiled emulsions were quite common in the early part of the century. Some of them were boiled for hours to achieve final sensitivity and contrast.
The boiling process was most normally done in the absence of ammonia itself, or very low levels. High levels of ammonia in single run ammonia digestion allowed the use of lower temperatures and shorter total making times.
The emulsion in this article also assumed the use of an active gelatin with enough allyl thiourea in it to give a degree of sulfur sensitization which increased speed.
This method was pretty much dead by about 1930, perhaps earlier. Wall makes reference to it in his 20s books but Clark only remarks on them as being older types in his books from the 40s.
Modern emulsions are made with a double run, accelerated growth technique which uses up to 25 steps to achieve very high speed and proper distribution of the iodide.
Slow speed emulsions are always possible with less fuss and bother, but it must be remembered that todays enlarging papers are about ISO 25, and so rank as high speed when compared to those in your reference. They would be about ISO 6 if coated on film. The emulsion in your reference might be the same speed or slower, but would be quite coarse grained if it goes as I suspect.
Thanks PE. What grabbed my attention was the emulsion color; looks very much "film" to me.
On that page, I saw no photo of the emulsion itself, or a plate, but basically an emulsion is an emulsion. A bromoiodide emulsion is a yellowish red, with the intensity varying with grain size and iodide content. A chloride emulsion is white, and a bromide is slightly yellowish.
Spectrally sensitized emulsions take on the opposite color of the region to which they are sensitive, thus a red sensitive emulsion is cyan, and an ortho sensitive emusion (blue and green) is red. A pan sensitive emulsion is gray.
There are spectra of these in Mees and James.
The author says it was blue-gray emulsion, and it was the reason I was curious.
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Sorry, I missed that part. IDK what that signifies in this case. Probably that there was appreciable fog on the grains to start with. These early emulsions were difficult to control and at that time, some were developed in developers containing ammonia.
Modern B&W film emulsions are gray or blue gray due to being panchromatically dyed, containing acutance dyes, and due to the AH dye undercoat. At that time, the emulsion would not have been panchromatic and would have been yellow or light orange if correctly prepared and fog and silver free.
I just remembered that there was a procedure whereby they looked for the formation of a series of different colors in samples of the diluted emulsion examined under transmitted light. This color gave them some sort of measure of the grain size and degree of ripening due to internal reflection changing as grain size changed. Different colors meant different grain sizes, but the actual bulk color of the emulsion was still yellow orange. This procedure was rather wide spread in the very early days of photography.
Last edited by Photo Engineer; 12-09-2006 at 01:43 PM. Click to view previous post history.
Reason: Adding an afterthought
It's too bad that those gelatins are no longer around; it would be interesting to try to reproduce this process and get an idea of what it was like to shoot an actual plate of the time.
On another note, would it be possible to add the necessary chemicals to current photo gelatins in order for them to be a good approximation of the ones at the time?
That question was already asked in another thread.
Excess halide retards the sensitization by these ingredients. Having sensitization carried out during precipitation introduces an extra variable into the making process.
Therefore, it is best to add these chemicals after precipitation and then if it an unwashed emulsion to add the chemical or wash and add the chemical.
The chemicals to be added include any one of the following; sodium thiosulfate or sodium thiocyanate, or thiourea, or allyl thiourea with or without a gold salt.
However, with the older emulsions, speed to be gained will only put you into the mid range of about ISO 40 or so at best. The contrast gain may be significant.
Boiling has another drawback, and that is the effect on gelatin. Hours of heating gelatin at near boiling temperatures causes it to denature and lose its gelation and hardening properties. In simple words it falls apart. It really is not a good way to make emulsions.
Please see the other threads on this same topic.