Volume 3, Chapter 25:
Methods for Strengthening and Protecting the Fingernails
The fingernails are
different from person to person in terms of hardness or softness. Tough nails can play with durability. Softer nails may be weak and easily
breakable. One technique involves green
ginger and bai-ji (白芨)
in equal amounts, sliced up, and boiled in water until thick. Take this and put it on the fingernails. Over time, the nails will gradually become
stronger. Or use
Some take goose quills and cut them up and use a little fish glue to stick the quill on the left-hand thumb border, thus preventing playing damage (to the nail). These are called “finger protectors” (護指).
 This is the tuber of hyacinth bletilla which is used in Chinese medicine and in some cases is applied externally. Both this substance and peppercorns may have bactericidal properties and some fingernail problems may be caused by a low-grade infection or fungus. A western alternative medicine for improving fingernails is called biotin which is a B vitamin and is ingested.
 It is likely given the context that the author means one applies the smoke to the nails, but the text is not clear. Toasted silkworms are used in Chinese medicine but they are ingested and are used for more serious disorders than bad fingernails. Still perhaps there is a bactericidal property here as well?