What about coriander seeds? I have some from last years planting which say that they can be sown until 2023 but they look a bit tatty. Should I risk it or use some freshly bought?

 Home made chilli and pilua rice 1/2 a baghette 

Would they be dangerous for the garden?

Anton:

 My job involves regulation of cleanup of hazardous waste sites, so I am familiar with many types of chemicals from that perspective.  "Solvents" can cover a lot of materials. I wouldn't be too worried about acetone or ethanol, for example, but I would be more concerned about other solvents, such as tetrachloroethene. Are there identifying marks on the barrels, with numeric codes (such CAS number), names, hazard warning labels, or transportation safety information related to the former contents?  If yes, photograph them, I may be able to help you figure out what was in the barrel.  I wouldn't be too worried about acetone or ethanol, for example, but I would be more concerned about other solvents, such as tetrachloroethene.

I wouldn't go sticking my nose in a barrel that formerly held an unidentified solvent, but if you detect a solvent odor when the barrel is open, it may not be completely decontaminated.

Many solvents are volatile, some could potentially evaporate if given enough time exposed to open air.  Others (especially petroleum solvents) may be tougher to get rid of.

Well it has had a first cut now, doesn't look brilliant, but as it is going to rain all week, I am pleased its done, now the edges.  Thanks for your help.