I'm sure you've heard that it's important to wait for a week or so before using newly mixed collodion. "It needs to ripen", they say. Well, what is this "ripening" thing anyway? And how do we know when it's "ripe"? While the answer to these questions depends a little bit on the formula used, generally, we can say that ripening is a real thing and it does have noticeable effects.
As collodion ages, we've all seen its color change from pale yellow to amber and eventually to dark red when gets really old. This color change is due to the iodides in the salted collodion breaking down and releasing iodine into solution. We can replicate this aging process in brand new collodion by intentionally adding iodine to it in the form of tincture of iodine. I did this in an experiment a couple of weeks ago and found some interesting results.
When small amounts of iodine are added to the collodion it takes on a light yellow coloring; we also begin to see an increase in the films contrast. As more iodine is added, the film's contrast continues to increase, eventually reaching a peak value when the collodion takes on an amber color. Eventuallly, the collodion color darkens with the continued addition of iodine and becomes increasingly red as the film's contrast begins a relentless decline for the remainder of its usable lifetime. In addition to this loss of contrast, the film's sensitivity also declines, and can drop by as much as two stops for very old and very red collodion.
Because the effects noted depend upon the concentration of iodine in solution, the collodion's color can be used as a relatively reliable gage for the collodion's "ripeness". Honey colored collodion is approaching it's peak performance while dark amber is the mark of a collodion that has begun it's long and gradual decline in performance.