A couple weeks ago I was checking out eBay for vintage natural whetstones and found an interesting item. The seller stated 'this is an unknown whetstone. It is not a Hindostan, though it feels finer. It feels similar to bisque. There are saw marks all around the edges. It's hard and dense, not crumbly. This stone would be fine for knives, chisels, plane irons, etc.'
Curiosity got the best of me so I placed a bid for $11.99 and forgot about it. Besides, I would probably be outbid. No loss. A week later I received an email stating that I was the winning bidder.
The game is afoot!
When the stone arrived it looked exactly like the photos and had a faint smell of oil. Did I just purchase another common Arkansas oil stone? I was beginning to feel disappointed. I decided to boil the stone in Dawn dish washing liquid. The stone didn't change color but the faint smell of oil was gone. I could see the original saw marks and the wear on the stone was minimal except for the normal nicks and chips found on a stone that may have sat in the bottom of an old toolbox.
I started flattening the stone on a DMT lapping plate and noticed that the color of the stone began to change as a fresh surface was exposed. Also, the texture did not feel anything like an Arkansas stone. The stone was very soft and felt smooth like fine chalk or clay and I could feel tiny quartz particles releasing as I continued to work it with the diamond plate. I decided to lap all six sides and chamfer the edges.
I began seeing interesting mineral streaks laced throughout the stone which confirmed this was a natural stone and not man made. Was this an Ardennes Coticule or a Belgian Blue?
Further research leads me to believe this stone to be a Thuringian!!
The softer Thuringian water stones were mined in the area of Steinach/Thuringia, Germany during the 1800's by small family businesses and many were sold to Escher Company which was located in Sonneberg.
Thuringian water stones are slate stones or more precisely mud slate. These slates consist of quartz - which is an abrasive material, clay, mica (glimmer) and chlorite. The softer stones were found in the upper-Devonian age deposits.
The hones came in different colors with yellow green, blue green, light green and dark blue. The color was determined by the amount of chlorite and other trace minerals in the stone. Escher would grade their stones by color and may have related to the speed which they cut. The lighter colored stones seem to cut faster. The more blue stones leave a finer edge and were preferred by barbers as razor hones and seem to be in the area of 12 to 15K in grit. The darker stones should be used with a slurry.
A quick search on eBay for vintage Thuringian hones show they sell between $70 - $400. Not bad for an $11.99 bid. The mystery of the unknown sharpening stone is solved my dear Watson!!