Nineteenth Century Salt Making – Tricks of the Trade

In the 1740’s the Admiralty was still buying French Bay Salt For provisioning the Navy even when at war with France. The problem was investigated by Thomas Lowndes of Middlewich and in his “Brine Salt Improved” of 1746 and “Letter of Advice to Brine Salt Proprietors” of 1748 he recommends a method of open pan operation that, in effect, became the common pan method of open pan working. Basically this was slow evaporation at lower temperatures in larger pans.

The Introduction of Additives

Thomas Lowndes also makes the first reference to the use of alum as a pan additive which produced hard clear crystals. Alum was still being added to common pans for this same purpose in the 20th century. Lowndes also describes the growth of large pyramidal “hopper” crystals, a common feature of the fishery salts and not observed with fine salts. Thomas Lowndes’s “ Improved brine salt” like common salt, did not require drying in a hot house but drained till sufficiently dry on the hurdles at the side of the pan.

Improved Extraction & Refinement

During the 18th century deeper brine shafts and improved pumping technology produced a clear brine and the use of blood, eggs and ale for the clarification of muddy brine became unnecessary. However at some time in the 18th (or maybe 19th ) century, it was observed that certain additives could produce small changes in the crystal size of the traditional fine Cheshire salt formed as lumps for hot house drying. Possibly it was observed that the residual foam from the addition of egg white or blood gave slightly larger crystals than when no additive at all was used. This would produce a lighter lump. A variety of additives are listed in 19th century texts such as butter and other fats, soap and glue. Salt makers no doubt had their “secret recipes” and the topic is not well researched. By the 20th century animal glue and soft soap were the standard additives.

In an open pan salt crystals form on the surface of the evaporating brine and if not disturbed grow in size until too heavy to float. They then sink to the pan bottom and the point where this takes place will depend on the surface tension of the brine. This will be increased by the glue and decreased by the soft soap.

The size of salt crystal affects the bulk density of the salt crystals and so glue which produced a larger crystal yielded a lower bulk density. Soap which produced small crystals would give a higher bulk density. Merchants selling to West Africa where it was sold by volume and not weight preferred a low bulk density while the fine higher bulk density salt produced by adding soft soap was preferred for butter and cheese making.

Under certain conditions the salt crystals on the surface of the brine would form as a continuous crust which reduced the rate of evaporation. This was known as “forming a set” and it was observed that the tendency to form a set was encouraged by the addition of glue and inhibited by the addition of soft soap. Part of the art of making salt was knowing how much glue or soft soap to add to the brine.

The method of making dry salt in the form of lumps by casting the crystalline slurry in a suitable mould has been a world-wide practice and was certainly adopted in Iron-Age Britain. From at least as far back as the 12th century and up to the 19th century the inland salt makers made dry salt by first casting the crystalline slurry in conical wicker moulds known as “barrows”. In the 19th century there was a gradual change to a square section tapered mould of elm wood called a “tub”. Various sizes of tub were used denoted by the number of dry weight lumps to the ton. The most popular size was the 80’s, each dry lump weighing 28 pounds. These were essentially a standard shape and size throughout the industry. Likewise hothouse design and practice were basically standard arrangement of raised cast iron covered flues with cooler ditches between. The lumps when first made were fragile and were stood upright in the ditches until strong enough to be handled. They were then stacked on their side on top of the hot flues until finally dry. The whole process took about two weeks.