This is due to a toxin stained.

这是由于毒素染色。

youdao

Litvinenko's coffin, a dark-stained Jacobean oak Garratt casket, was due to be driven around the perimeter of the cemetery before being laid to rest at the burial site in a non-denominational service, the UK's Press Association reported.

CNN: Wife, 50 friends at ex-spy funeral