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.