I have it! I woke up quite suddenly with the answer plain in my head.
For several weeks now I have been thinking of every method I might use to measure the hardness of a hardened block of wood. Most of them have involved using some kind of clamp or vise with a screw that I might tighten or loosen. How many turns would it take before the jaws of the clamp mar the surface of the block? I could repeat this trial periodically, looking for changes.
The difficulties here are obvious. First, it is quite difficult to ever mar the surface of a hardened object. That is one of the reasons why hardening is a mainstay of my craft. Second, tightening the clamp is exactly the sort of action that would cause the block to collect effort; and my trial is precisely to see how long it takes the block to become fragile before that happens.
And that, bluntly put, is the key. That is my epiphany—that I am making everything—hah—harder than it needs to be.
It is easy to tell when fragility has set in; Mme. Poquêrie brought me a plate that I was able to crumble between my fingers with no great amount of force. Many of the plates that were returned to me could be marred with a finger nail.
So I have no need to measure hardness. What I must measure is time. Given a newly hardened block of wood and a lifting block capable of suspending a particular weight, the two placed in close proximity, how long will it take before the block becomes fragile? It is so simple. I have asked this question of myself a hundred times at least, and especially when Luc returns from Marc's farm and says, "It is all just the same as yesterday." But I was so focussed on the degree of hardness remaining that I did not listen to myself.
But time is the key variable. First, so that I know how long a hardened object may safely be used, as part of a sky-chair or boat; and second so that I may learn how to extend that time.
Luc tells me that he had always planned to measure the length of time. "For the plates were quite hard until they started breaking," he said. Not for the first time it has occurred to me that my apprentice may be smarter than I am. I console myself with the thought that perhaps he has had a more congenial teacher than I did; and also that it was I who came up with the notion of using a balance to measure the degree of lift in a lifting block.
The best part of this discovery is that our work to date will not be wasted. We know when we set up our trial in the shed on Marc's farm; and once the hardened block begins to crumble I can bring the lifting block into the shop and measure how much weight it can suspend.
I wish we could run multiple trials simultaneously; but they would have to be wildly separated. Perhaps the next step is not to try again with a stronger lifting block, but to move the hardened block and the lifting block apart. There must be a distance beyond which the two blocks no longer affect one another. But hah! How will I ever know if they are far enough apart? All I can see is that the hardened block has not yet begun to crumble. If they are far enough part, it never will. I do not believe I can wait that long. We shall have to make some trials in that direction…but perhaps there is a farmer on the far side of Bois-de-Bas from Marc who would let me set up a shed. How ever large the distance must be, I am sure that it is well under a mile!
It is quite early…dawn is just creeping into the sky. I believe I shall put out my lamp and return to bed and see what sleep I might get before the cutters resume their work on the town hall grove.