White Stains in Ambrotypes: Possible Causes

I recently made these ambrotypes and noticed the white stains or blemishes circled in red on the attached images. I'm wondering what might have caused this artifact. Could it be due to an issue with the collodion coating, the development process, or something else? Since they appear thicker (probably) and whiter, it seems like it could be a surface irregularity caused by overly thick collodion although I am not sure. These artifacts don’t happen every time, but about once every 30 to 40 plates. While I find them aesthetically interesting, I'd love to hear your thoughts on the possible causes, especially given their whitish appearance. Thank you!!!

When they do appear, do you clean or otherwise try to correct the next images?

What effects do you see if you do ANYTHING?

D_Quinn said:

Could it be due to an issue with the collodion coating, the development process, or something else?

Click to expand...

This appears to be fog deposited by contamination of something. This 'something' could either be the developer itself, or perhaps also the plate itself before collodion is poured onto it. It's possible for instance that some contaminant sticks to the edges of the place and as you pour collodion onto the plate, it's carried on the surface of the collodion across the plate as soon as the collodion hits the contaminated edge. This is the mechanism through which 'oyster shell' marks appear to emerge, and this looks very similar - although these extend further into the image than typical oysters. This might be due to a contaminant that spreads easier/quicker than the whiting/calcium carbonate typically responsible for oysters, or a contaminant that's present in a larger quantity.

Thank you for sharing your thoughts!

I always wipe the glass thoroughly each time, but contamination might still be a factor. I noticed this pattern at the fixing stage, where the unexposed silver was removed. (There was nothing particularly unusual when I applied the collodion or after the development process.)

I remember that even a cotton ball couldn't remove it during the washing process. Well, I understand that many factors could be involved, so identifying the exact cause is difficult. In any case, both instances happened on hot days (28°C room temperature and 60% humidity).

If the same thing happens again, I will try to investigate more thoroughly.

These are classic "Oyster marks". It's contamination of the plates by exposure to exposed (to light) silver iodide/bromide left in the plate holder from previous plates.

Thanks for your comments! I guess I need to wipe my holder much more carefully after each shoot.