They went down, wetted the sand, and ground away for fifteen minutes; washed the glass, started again, and at the end of another fifteen minutes went up to repeat the process of drawing off the thick water into the basin. This was left to stand till evening, when the water was poured back, and about a double quantity of thin paste to that obtained in the morning placed in a size larger bottle, and labelled “thirty-minute emery.”
Again the whole was well stirred, and left for fifteen minutes; the process repeated, and a much larger quantity obtained and bottled.
The next day the emery was stirred, and allowed to settle for five minutes; then for two minutes, and the remainder bottled by itself, this being by far the largest quantity, and in fact so much strong sharp grit.
“There!” cried Uncle Richard; “now, going backwards, we have six different grades of material, beginning with the coarse, and going up to the fine sixty-minute powder or paste for polishing, for these things have to be made exquisitely fine.”
At the next attack upon the glass to dig it out into a hollow, the sand was all carefully washed away, showing the disc to be thoroughly scratched all over, and looking somewhat like the inside of a ground-glass globe.
“So far so good, Tom,” said Uncle Richard; “now let’s try our mould.”
He took down the convex-shaped piece of zinc, and placed it upon the newly-ground-glass, into whose face it descended a little way, but only a very little.
“Not deep enough yet, Tom,” he said; “the mould ought to fit into it exactly.”
“Yes, I understand now,” said Tom; “we have got to grind more out of the middle.”
“Exactly.”