Saltmaking in Britain has always been different from our more southerly continental partners, since we are too far north for reliable large-scale solar evaporation. Yet we do have major resources of peat, wood and coal, so UK salt making tends to rely on fuel rather than on the sun, or on a combination of the two to evaporate brine.
Britain forms a 'hinge' between southern European solar and northern European thermal saltmaking, so a number of different evaporation processes have been carried out in UK saltmaking.
These different processes need very different environmental settings, leave very different field traces and archaeological evidence are often not clearly distinguished in historical accounts published in documents and books.
In the literature, there is much confusion about coastal saltmaking. The ECOSAL project has a lot of work to do to establish what techniques were really used, where and when.
From our research to date, we know that the main processes in British saltmaking have been:
Solar - As on the Continent, seawater was run into a set of very shallow ponds to concentrate by evaporation to a strong brine. This was then run into smaller ponds where the salt crystallised out.
Partial Solar - Seawater was concentrated by evaporation, but the brine was then lifted or pumped into panhouses, where it was boiled to crystallise the salt.
Sleeching - The salt-encrusted surface of saltmarsh silts was scraped off, the salt content leached out, and the resulting strong brine boiled in a ‘saltcote’ (a small building normally containing lead pans fuelled with peat or wood).
The panhouse process - Seawater was boiled in panhouses consisting of large iron pans built into a brick or masonry structure with built-in flues, and almost always fuelled with coal. The process could be controlled to produce different types of salt - rapid boiling produced fine-grained salt suitable for buttermaking, whereas slow simmering produced large crystals for fish curing.
Salt refining - Impure salt (most often rock salt) was dissolved in seawater to form a strong brine, the impurities settled out, then boiled in a panhouse to produce clean white salt.
Other methods known in northern Europe may also have been used. For example, in the Low Countries, Roman and Medieval saltmaking was by 'selnering' (burning salt-impregnated peat, leaching the salt from the ashes, and boiling the strong brine). In Denmark and Holland, eelgrass (Zostera species) was burnt and salt was leached from the ashes to make a strong brine and then was boiled.
All of these saltmaking processes can be grouped into three main types:
Solar - evaporation with no use of fuel
Direct boiling - boiling of seawater without any prior concentration
Pre-concentration - removal of water or addition of salt to convert seawater to a concentrated brine, before boiling.