The cartwheel twopenny coin was not struck again; much of the mintage was melted down in 1800 when the price of copper increased and it had proved too heavy for commerce and was difficult to strike. Much to Boulton's chagrin, the new coins were being counterfeited in copper-covered lead within a month of issuance. Boulton was awarded additional contracts in 1799 and 1806, each for the lower three copper denominations. Though the cartwheel design was used again for the 1799 penny (struck with the date 1797), all other strikings used lighter planchets to reflect the rise in the price of copper, and featured more conventional designs. Boulton greatly reduced the counterfeiting problem by adding lines to the coin edges, and striking slightly concave planchets. Counterfeiters turned their sights to easier targets, the pre-Soho pieces, which were not withdrawn, due to the expense, until a gradual withdrawal took place between 1814 and 1817.