Watt's letters to Murdock are most particular in details, especially as to screws, nuts, and tubes, with strengths and dimensions, always illustrated with pen-and-ink drawings. And yet all this was done merely for mechanical amusement, and not for any personal pecuniary advantage. While Watt was making experiments as to the proper substances to be carved and drilled, he also desired Murdock to make similar experiments. "The nitre," he said in one note, "seems to do harm; the fluor composition seems the best and hardest. Query, what would some calcined pipe-clay do? If you will calcine some fire-clay by a red heat and pound it,--about a pound,--and send it to me, I shall try to make you a mould or two in Henning's manner to cast this and the sulphur acid iron in. I have made a screwing tool for wood that seems to answer; also one of a one-tenth diameter for marble, which does very well." In another note, Watt says: "I find my drill readily makes 2400 turns per minute, even with the large drill you sent last; if I bear lightly, a three-quarter ferril would run about 3000, and by an engine that might be doubled." The materials to be drilled into medallions also required much consideration. "I am much obliged to you," said Watt, "for the balls, etc., which answer as well as can be expected. They make great progress in cutting the crust (Ridgways) or alabaster, and also cut marble, but the harder sorts soon blunt them. At any rate, marble does not do for the medallions, as its grain prevents its being cut smooth, and its semi-transparence hurts the effect. I think Bristol lime, or shell lime, pressed in your manner, would have a good effect. When you are at leisure, I shall thank you for a few pieces, and if some of them are made pink or flesh colour, they will look well. I used the ball quite perpendicular, and it cut well, as most of the cutting is sideways. I tried a fine whirling point, but it made little progress; another with a chisel edge did almost as well as the balls, but did not work so pleasantly. I find a triangular scraping point the best, and I think from some trials it should be quite a sharp point. The wheel runs easier than it did, but has still too much friction. I wished to have had an hour's consultation with you, but have been prevented by sundry matters among others by that plaguey stove, which is now in your hands."