The Calotype process

The Calotype process was a positive/negative process that wasn’t introduced until 1841 and stayed popular for the next ten years. The process was introduced by Fox Talbot who devised the process and showed his results at the Royal Institution in 1839. When using the term Calotype it strictly refers to the negative image, but commonly it is taken to mean both, positive and negative. A piece of paper was brushed with a light salt solution, dried, and then brushed with a thin silver nitrate solution, then dried again, creating silver chloride in the paper. By doing this the paper becomes sensitive to light and is then ready for exposure. Doing so takes about a half hour for a print our image look. In 1844 Fox Talbot opened a photography establishment in Reading in order for him to mass produce his images. In order to make a print the negative was placed on top of more photo paper, laid down flat in a glass frame and then allowed to develop in the light of the sun.
http://www.rleggat.com/photohistory/history/calotype.htm