My 1960 World Book Ecyclopedia says the following:
“ICHTHYOL is the trade name of a drug prepared from certain hydrocarbon or mineral shales. It is a dark-brown, thick liquid. Ichthyol gets its name from the scientific name for fish, because the shale contains fossil remains of fishes. Ichthyol is prepared by distilling the shale, treating it with sulphur, and neutralizing it. Ichthyol, which is really ichthyolsulphonate, is used as an antiseptic and as a soothing, skin preparation. It is also used in treating boils.”
Yup, someplace toward the end of my Gramma’s time the “canth” part of the original word got dropped, probably for convenience. Canth implies a sea shell connection so, yeah, that would relate to oil bearing shale. Gramma died in 1966, before any commercial concoctions were readily available or I expect the family druggist would have suggested one instead of making her mix her own.
That ancient fish would have been extremely flat by the time it got boiled down for Gramma. And that’s why I never throw books away. Sometimes they have stuff I wish I knew sooner or later.