In his 1736 publication Smegmatalogia, or the Art of Making Potashes and Soap, and Bleaching of Linens, James Dunbar describes a process for the preparation of potash. The intended user of the process was the common Scottish farmer. Dunbar was anxious to imbue the common Scot with the ability to “bleach” his own linens. It is important to realize that the meaning of the word bleach in the early 18th century is different from contemporary use. The modern use comprises notions of decolorization through oxidation of color bodies to produce a white appearance. The 18th century concept involves the apparent cleansing and subsequent lightening of a fabric.
The book begins by detailing the preparation of a solution or extract from ashes called Lee. To obtain this solution, the “Country-Man” would carefully collect Scottish vegetables such as the wood of oak, ash, beech, “thorns”, juniper trees, and “whins”. Suitable herbs included fern, breckens (or brackens), wormwood, thistles, stinking weed, and hemlock.
Dunbar is careful to instruct that the vegetation should be burned in the shelter of a house but in such a way as to avoid burning down the house. The purpose of burning the vegatation in a shelter is to avoid having rainwater come into contact with the ashes. My interpretation of this is that runoff carries away soluble potash.
The ashes are placed in a container and covered with water. The ashes are soaked in water until such time that the Lee “carries an egg on its surface”. What Dunbar is telling us is that the extraction of the ashes needs to go until the worker obtains in the solution a particular specific gravity- this is a specification. There is some minimum specific gravity of the Lee that will float an egg. And the higher the specific gravity, the more volume of the egg rises from surface of the Lee. The specification herein is required for the next operation. In order to carry out a successful saponification of tallow, the Lee solution must be sufficiently concentrated.
Dunbar then describes steps where the Lee is combined with the ashes of ash, beech, or fern followed by boiling the water off to afford “thickens of pottage“. The residue is shaped into balls which are then calcined in a fire to afford a substance that may be stored in a dry container for the purpose of making soap.
The discovery of chlorine in 1774 by Scheele and the subsequent of discovery of chlorine bleaching by Berthollet gave us our modern conceptual notion of bleach and bleaching. The develoment of bleaching powder was made by Scottish chemist Charles Tennant who took a patent in 1799. Tennant’s associate, Charles MacIntosh, is thought to be a contributor to this invention. Bleaching liquors and powders soon became an important raw material for the bleaching of paper and fabric.
The procedure described by Dunbar is a chemical process. It tells the user when the extraction is complete, qualitatively at least, by a folksy means of specific gravity determination. This is really very clever- it uses a common object to do the test and the result is readily apparent. Bleaching in the early 18th century involved the use of soaps and of urine treatment and bleaching fields- a far cry from what we now think of as bleaching.