A previous post noted work in progress by the National Society of Consulting Soil Scientists to develop standards for biochar testing similar to those for compost testing. You can have your bathtub char tested now. The value of the advertized tests is for determining "quality of char for agriculture and carbon credits". Tests inlcude:

• Parent material - Water, Matter, Carbon, Nitrogen, Ash
• Sample Chared - Water, Volatile (HO,C, N), Fixed (HO, C,N) Ash (Acid Soluble, non-soluble)
• Optimum Char - Fixed (HO, C, N), Ash (Acid Soluble, non-soluble)

The tests for water and volatile components may be particularly useful due to concerns about charcoal spontaneous combustion during shipment and storage. Under some conditions exposure to air and water can cause exothermic reactions with some char components, and set off a chain reaction where the heat is not dissipated, which increases the reaction rate, and can lead to open flame.

The same lab does compost testing and soil testing.

Testing is useful - information is good - but it may be worth noting that there is scientific controversey about the way tests are done and the interpretation of the results. It's never cookbook style - just get a recipe and follow it - as we might expect from complex systems that are only partly understood. That's one of the reasons that there is a market for consultants, lots of words shared among practitioners, and variable results.