Not exactly acquisition of the week as I bought this at the June Spink sale - a Bristol 1644 B1 halfcrown. I bought it to sell on but is too interesting to dispose of as it appears to be pretty much all copper with silver surfaces, but all the indicators are that it is genuine and struck using metal from the bottom of the pot. The dies are definitely ok and the ring suggests a rolled sheet as opposed to a cast, the weight at 15.05g is spot on. The surfaces have presumably been pickled using an acidic tartrate solution to restore the silver at the surface. Copper halfcrowns are unlikely to have been popular, but presumably the weight of metal in the pot dictated a finite number of coins be produced from it, give or take a couple
Notwithstanding the pitting to the surface on the horse which might have indicated a cast, there are no instances I am aware of where Bristol halfcrowns have been counterfeited. The angled edges of the coin are spread out creating a recessed centre to the edge which effectively would act as a security edge and eliminate a casting because it is not cost effective to cast a single coin from a mould.
We know that Thomas Bushell was sending about £100 of silver every week from his mines near Talybont to Shrewsbury, Oxford and then Bristol. We know from the records (shown in Boon) that the silver ingots supplied from Wales were of higher purity than the standard, thus requiring copper to be alloyed in order to get the correct fineness. The preferable use of sequestered touched plate meant that the latter silver could be coined without needing to alter the fineness.
We also know that copper's melting point is about 120 degrees higher than that of silver, so if the silver is melted in the pot and the copper added afterwards, there is no guarantee that the temperature would be high enough to fully melt the copper. If the copper was melted first and the silver added, then you would be sure to get good mixing.
The question of melting points may also explain why you rarely see haymarking on gold compared to that seen on silver, as the mp of gold is only 20 degrees below that of copper.
All good fun.