This particular Pyrenees stone has been shaped with this tool, keeping the stone’s 12″ length neatly aligned with the plate’s 11″ length, and this results in a curvature across the 12″ length of a bit over 3mm upon one of its two ~2×12″ honing surfaces. You’ll receive the exact stone pictured.
Across the 2″ width of this side of the stone, the stone is thicker in the center by about 0.33mm in a nice smooth arc going across the width, for an excellent bevel-setting specialist, to be then refined with a pasted strop, a flat hard dense stone appropriate for finishing, etc. Use with oil is recommended by the French.
You can buy it with a resurfacing plate together for a special price, and I would recommend if you’re taking on old eBay beaters to get a bevel-thinning specialist stone, such as a 800 to 1000 grit Japanese waterstone, and shape that stone toward the 9″ axis of the special lapping plate. Doing so will make a stone with a much shorter diameter than you’re getting here, which is quite useful for imparting a nice thin shape to the rear of a razor’s bevel. If you’re coming from something that was ‘bevel set’ to be a completely flat isosceles triangle form (which is NOT what the razors makers intended in the Victorian Era!), stay on such a shape as your razor’s bevel becomes wider, with new scratches from your short-diameter-specialist thinner. Allow those scratches to get to at least 50% of the width of the bevel’s span, and then switch to this prepared surface, stay on that until it overtakes the whole edge (which will not take long because you’ve already made it so thin!), and then top off this prepared bevel with the fine/hard/flat finishing stone of your preference, or upon a strop covered with diamond abrasive, Dovo red/black, chromium oxide, etc. You’ll be amazed at your razor’s behavior!
A copy of this large sharpening stone which shown to me eleven years ago by the former managing director of Thiers-Issard, who was the first to explain to me of the concept of the wheel-shaped stone. According to that suave fellow, following the hollow grind procedures, TI first establishes a cutting edge on a pair of wheels seen here (which is done upon convex abrasives with a significantly shorter effective diameter than what you’ll get off of imparting just ~3.25mm of rise over 300mm of length), and then they move on to this natural Pyrenees stone for a bit, which (- again, ~12yrs-old info, but highly unlikely to change) they shape by hand to be slightly convex (and too subtle for you to see in that video, but I assure you a ruler placed on the stone in-person would see it), and finishing via a strop regimen as here. Their idea is that by thinning the bevel form and only leaving a small piece, the work with the bench stone (either flat or very long diameter) only has a tiny patch of metal upon which it can work, and then refining it with the slightly concave surface of a pasted strop ensures they are polishing at the very apex of the bevel form. That is why literature included with a Thiers-Issard razor will state that the new razor should not need a stone for many years, only a pasted strop, because they believe that the new razor’s bevel should be so thin that it would take many years of use to make its tip too thick for paste-sharpening to return the razor to comfort.
I don’t know their particular numbers of effective diameters they use to impart their preference toward the razors’ bevel forms. But as this is a coarser natural stone appropriate for bevel-setting a straight razor or finishing a kitchen knife, in my professional opinion for the use with straight razors it benefits from a reduction of thickness of not less than 2mm over this stone’s ~12″ length.
I was told 12yrs ago by the old director that TI used this stone with new, clean low weight motor oil. He did/does have a thick French accent, so perhaps something was lost in translation, but ‘motor oil’ were the words I remember. I am told by a French customer that there is some sort of oil at hardware stores that they refer to this way, but isn’t actually meant for going in a vehicle/engine. As I’ve no intention of ever reselling my piece of this once I get its surfaces prepared geometrically to my own standards, motor oil I shall use!