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Joe,
The Eschers are waterstones. They are naturally quarried stones from Germany but the original source is long gone. When they turn up on eBay they make the price of a Belgian yellow look like...

Joe,
Thiers Issard confirmed to me that they at least use a Belgian yellow along with "naptha" which after some research equates to, well, kerosene. They felt it clears the swarf better leaving less...