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Also Water Glycerine is a potentially substitute solution to get a feeling how honing on oil would be....

...depending how the mixture is set up you can simulate the viscosity or thickness of an...

Would have guessed Sandstone too...

Use a fine WD paper 1000grits and rub it over the side where both layers come together....just check which colors appear...

I know that you and the owner are well experienced, but i have to ask if this might be a very dirty or miscolored Coticule ?