[pure-silver] Re: Gelatin filters

Frank Filippone wrote:

I may be wrong, but I thought Gel filters, as now made,  are actually
polyester or something that has nothing to do with gelatin, but he name came
from the older manufacturing process that did use gelatin.
Well, the "classic" Kodak Wratten filters were certainly made with gelatin, the reason being that the dyes could be added in an aqueous solution without having to heat the gelatin, which could otherwise affect the dye. This allowed for very precise control of the absorption spectra for the filters. Up to the invention of the Wratten filters, it was quite common in scientific and technical photography to use dyes in acqueous solution, held in a cuvette in front of lens - possible in the lab or studio, but impractical in the field.
Adding dyes to optical glass requires the glass and dies to be heated to the melting point of glass, which seriously limits the possible dyes which can be used, and can also cause significant batch-to-batch variations.
As I said, earlier, I also believe the Wratten filters were protected by a thin layer of lacquer on each side, which would provide some protection against absorbing humidity from the air. 


Today's filters do not degrade "melt" in wet area use, which I thought would
be a big issue in natural gelatin....
Absorbtion of water is certainly a problem with gelatin in high humidity situations, as everybody on this list should know. Once again, I believe the lacquer provided some protection, but as I recall, a drop of water on a gelatin filter would spoil it.
There are certainly modern filter types made today which use various kinds of optical plastics. Off the top of my head, I don't know what materials are used, however. 

- Helge Nareid

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