The origin of Britannia standard (1687 - 1720) and optional afterwards is interesting and a bit humorous: silver makers melted silver sterling coins to produce silver items because the profit from selling silver objects exceeded the value of the coins. Eventually, it led to significant decrease of silver coins in circulation and the Royal revenue decided to introduce Britannia standard at .958, i.e. higher than sterling purity. The legislation between 1687 — 1720 forced silver makers to buy silver ingots and add copper to make .958 purity. In short, they could not melt sterling silver coins any more because higher standard was demanded and assayed. The objects are very rare (Queen Anne period) but Britannia standard was used afterwards, too for high — end products mainly. It is nice to have examples of Britannia standard silver of later date (as I said — 16th and early 17th silver is rare). Just have in mind that this is the highest purity ever used in silver and also think of the fact that silver alloy at that purity is very soft and easily damaged, bent, indented etc. Handle them with care and cherish them. The cleaning is the same as sterling silver, although I found a slightly different patina on Britannia standard in smoking homes — there was less copper in the alloy.
Ivan