Briquetage technically refered to the very coarse pottery used to make pillars and evaporation vessels used in an ancient method of producing salt by evaporating brine over fires. It is also used to refer to the manufacturing process which created hard, compact salt cakes which were removed from the pots by breaking them to extract the salt. This left behind characteristic pottery fragments and hearth debris used by archaeologists to identify these prehistoric and Roman-era saltern sites.
According to Salt: a World History the Roman empire expanded to the East coast of Britain around 43 AD they first found locals pouring brine on hot charcoal and scraping off the salt that was formed. They proceeded to teach them how to make brine in earthen pots by boiling it down and then breaking open the vessels to reveal the salt cakes. As the Romans moved north along the coastline they found that this ancient method of manufacturing was not unknown to the locals and the latest research shows salt water production in clay pots dates back to at least 3,800 BC in Britian. The practice continued well after contact with Rome and multiple salt works existed across the isles.
The UK Salt Association shares that "There have been extensive finds of Iron Age briquetage in the Lincolnshire and East Anglia Fenlands and along the Essex coastline. Here the sea water was concentrated in pottery pans 60cm wide, 120cm long, and about 12mm thick.
The strong brine was then evaporated in small pottery vessels supported on pillars to give the lump of salt which was obtained by breaking the vessels. "