- PREHISTORY
OF PHOTOCHEMISTRY
Not too many
years after Friedrich Risner carted his portable camera obscuras across
the countryside, Dutch scientist Angelo Sala began experimenting with curious
substances called silver salts. In a pamphlet published in 1614, Sala noted
that when powdered silver nitrate is exposed to the sun, "it turns
as black as ink." It is significant that Sala never distinguishes
whether it is the sun's light or its heat that produced the reaction. It
wasn't until 1725 that Johann Heinrich Schulze, a professor of anatomy
from Nuremberg, solved the mystery.
While attempting
to make phosphorous, Schulze discovered a chemical that created the opposite
effect. He noticed that a piece of chalk dipped in silver-tinged nitric
acid turned purple when exposed to the sun. The unexposed side remained
white. At first he thought this was due to heat, but trials by fire produced
no photographic effect. After a number of experiments, he discerned that
a solution of silver nitrate exposed to light turned black. In dramatic
demonstrations he used stencils wrapped around bottles of silver nitrate
to create crude photographic impressions. The Imperial Academy at Nuremberg
was amused but not overly impressed by his exhibition, and Schulze did
not pursue his study.
Espionage and
the need for covert communications advanced photochemistry to the next
level. In 1737 Jean Hellot, a Frenchman associated with the Academy of
Royal Sciences in Paris, proposed a method of secret writing by photochemical
methods. Using weak silver nitrate solution as ink, letters composed by
dim light remained invisible until exposed to sunlight for a few hours.
Images made from
silver salts were far from permanent. In 1777 Carl Wilhelm Scheele, a Swedish
chemist, discovered that blackened silver chloride was insoluble in ammonia.
But silver chloride that had not been exposed to light dissolved in ammonia.
This suggested that an image made from exposed silver chloride could be
"fixed" or made permanent by washing the unconverted silver away
in an ammonia wash.
Fascinated by
the relationship between light and photo-sensitive silvers, Scheele continued
to make important discoveries. Exposing a sheet of silver-chloride coated
paper to the solar spectrum, he noticed that some rays blackened the paper
more quickly than others. Violet, for example, turned the silver dark almost
instantly, whereas red took nearly 20 minutes. This observation would play
a vital role in making accurate photographic exposures.
Photography's
further metamorphosis owes a small debt to the butterfly. Tom Wedgwood,
son of the famous potter Josiah Wedgwood, was an avid botanist. He collected
specimens from across the natural spectrum, but was frustrated by their
ephemeral quality. He wanted a permanent record of his collection.
In 1796 he unknowingly
picked up where Scheele left off, and began experimenting with silver salts.
Coating paper and leather with silver solution, he pressed leaves, fibers
and butterfly wings against the sensitized surface. The resulting photo
grams were in one sense the world's earliest photographic images. Unfortunately,
they were far more delicate than even the most fragile butterfly wings.
Had Wedgwood known about Scheele's research, he could have fixed his pictures
with ammonia. Working in vain with a variety of soaps and varnishes, Wedgwood
abandoned his pursuit in 1802.