The black gunk referred to earlier by Wayland, and also used by my nan, was probably icthammol - a drawing salve. It is aluminum bituminosulphate, distilled from high sulphur oil shale and rumoured to have antibacterial, antifungal and anti-inflammatory properties. I can testify to its ability to bring splinters and glass shards to the surface. That said I've not seen it for about fifty years.
Zam-Buc is a pretty effective proprietary substitute and all round 'healing compound' still readily available which I use as a cure-all for any skin complaints including cuts, grazes and splinters.