All of this talk of daguerreotypes and ambrotypes and albumin prints is taking us perilously close to a discussion of photoprocesses and how they work.  So, perhaps, it would be best to begin with a discussion of tanning lines.  Everyone is familiar with tanning lines.  Stay out in the sun too long and you get a pale shadow – a negative image of your watch, your hat, or your bathing suit.

What is going on here?  Well, obviously, some of the skin is protected from the sun and doesn’t tan.  The rest is hit by the sun, mostly the UV rays, and does tan.  But what are the underlying biochemical processes?  There are two tanning mechanisms or chemistries.  The human body produces a protein called  melanin, which is designed to absorb and therefore protect the skin’s DNA from UV radiation.  There are two photomechanisms involved in the tanning process.  In the first, a rapid response, the light causes the melanin to oxidase and darken.  The second is a slower process whereby the body detects UV damage to the DNA – technically the production of pyrimidine dimers.  In response to this damage, the body locally produces more melanin and therefore absorbs more light in the exposed areas.  Note that in both cases light energy causes a chemical change that results in increased light absorption or darkening.

Tanning is a classic example of a solar gram.  You can achieve the same effect with a piece of colored construction paper.  Place a few opaque items, like keys or coins on the paper and put it in the sun for a few days.  The sun causes a chemical reaction that bleaches out the color, leaving shadows of the objects behind.

This is all fine and dandy; but there is a big problem.  If you show someone your beached paper or tanning line you are exposing it to sunlight and eventually it too bleaches in the case of the paper or tans in the case of the skin.  Early experimenters in photography had a similar problem.  They had all sorts of chemical reactions  that were caused by light, but they needed a way to fix the image – to remove unreacted  photosensitizer.  How they solved this problem is the key to the story of early photography.