We were recently asked to answer a question regarding whetstone repair: how to repair a whetstone that has developed a hairline crack all the way through the stone. The sharpener has been using oil on the stone and has a preliminary question of how to clean the oil out of the crack. The stone in question is a Belgian Coticule stone.
Note: With its rectangular shape and straight sides, this is not the type of stone we would recommend for use on a European scythe blade. The curved shape of the European blades mates well with an elongated oval stone.
We have no experience repairing stones here at Scythe Supply, but we do clean our stones often with white vinegar. We use about a tablespoon of white vinegar in about a half cup of water, soaking our stones for about an hour or less on an as needed. The vinegar helps clear the stone pores of accumulated grit and renew the tooth of the stone. As to whether or not this method will draw oil out of a hairline crack — unknown, but perhaps worth a try. A search of "remove oil stain from concrete" might turn up some other ideas, though I'd caution against using any chemicals as the stone will have direct contact with skin.
We use water and, occasionally, white vinegar on our stones. Natural veins of discoloration in whetstones are common. Any damage, in my experience, results in a complete break, typically when the stone contacts asphalt or concrete. Small chips or scrapes in a fine or medium grit stone can be smoothed down with a dry or wet coarse stone (this task must be done outside while wearing a mask; rock dust is extremely harmful to lungs).
As to repairing the crack in question, a suggestion of drilling and epoxying pins into the stone to support and arrest the fracture is being considered by the stone's owner. I don't have any experience with stonework, so I'm not sure how the stone will react. Maybe it will work, or perhaps the vibrations of the drilling will further the crack or develop new points of weakness. There's one way to find out.
We welcome your educated suggestions on either point: removing oil from a whetstone, or how to repair a cracked stone. Thanks for reading.