The best source I have is the book, 'A New history of the Royal Mint' by C Challis. Unfortunately it is out of print, but here is a relevant quote: "On 22 July Fremantle proposed in a report to the Treasury that the operations of coinage should be suspended from 1 February 1882 and that, so far as proved necessary, the remainder of the year should be devoted to the reconstruction of the buildings and the renewal of machinery. By the due date a stock of £489,000 in silver coin had been built up, concentration on silver being made possible by the placing of a contract with Heaton's for fifty tons of pennies, halfpennies and Farthings." Even better would be the Royal Mint's annual reports from 1881 and 1882, but my local library certainly doesn't have those. Possibly the larger libraries in London would have them.