Quote:
Originally Posted by haziz  Any way of replacing the cadmium with another cation and keep the formula cadmium and extra ether free.
Full formula please.
Thanks.
Sincerely,
Hany. |
The various cations have little to no impact on the speed or contrast of the image. Rather, the iodine and bromine quantities and ratio are important. However, the cations do impart some properties to the collodion that may or may not be desirable. Generally:
- cadmium salts thicken collodion and improve shelf life of salted collodion, however, cadmium is a toxic heavy metal; cadmium-salted collodions may be more fragile and also lift from the surface of the plates more readily and so may require an albumen subbing treatment to improve adhesion;
- potassium bromide requires some additional water to dissolve and easily precipitates out of the collodion; It imparts a longer clearing time to freshly mixed stocks; It works so well that I suspect it somehow balances the optimum water and dissolved salt content of the collodion; If any bromide (NH4Br, LiBr, NaBr, CdBr2, etc.,) is present in the mix along with potassium iodide, potassium bromide will form and precipitate from the mixture;
- ammonium salts dissolve readily and are useful when quick-clearing formulas are desired for immediate use; however, these collodion formulas have a limited shelf life and the contrast characteristics will change more rapidly than when using other salts (which is not a problem if it is used up quickly); ammonium salts may generate fumes that some find irritating;
- sodium salts are relatively inexpensive and can be used but they tend to promote rapid dessication of the collodion film and this may induce drying artifacts on plates;
- lithium salts also work but I haven't had the opportunity to shoot with them yet. I just mixed a batch last week and plan to experiment later this week. I suspect the collodion will perform well although its shelf life may be limited;
- zinc salts are found in some antiquated formulas but I don't know of anyone actively using them today.
You can substitute one salt for another in the formulas but to get similar speed and contrast, the molecular weights should probably be considered when substituting salts. Practically, it won't matter much which ones are used as long as the iodide to bromide ratio is ideally about 4:3 for ambrotypes and 5:2 for negatives, and the iodide content is about 1.1% by weight. Bromides increases the spectral sensitivity into the green while iodides add speed and contrast up to a point. An all-iodide collodion is going to be faster and higher contrast than one with some bromide in it. As any collodion ages, the contrast generally increases and the speed decreases.
As far as specific formulas, the one you've obtained on the collodion forum is probably as good as any. However, I would again caution that a low-ether collodion may be very fragile.
Joe
Linear Mode
