Carbolic soap, purchased at Beamish Open Air Museum, local to me.
Bought some last year and it is the only thing that keeps a persistent rash under control, following an insect bite three years ago.
It is difficult to cut and shatters into shale. This can be reconstituted but is a bit of a pain.
I have tried very sharp knives of various sorts and ended up using a junior hacksaw, far from ideal.
I was advised to use an heated knife but wary of damaging expensive knives. Then read that it can be warmed in the microwave oven and softened, making it malleable and easier to cut.
An oven time of fifteen seconds was recommended but proved insufficient, more like five minutes, done in fifteen second steps.
A bit of MDF, not the proper chopping board, and the trusty breadknife. All is well except the soap-flavooured sandwiches.
I will buy a cheap knife to heat up and try but have cut sufficient for about a year.