What the New Scientist piece doesn't mention is that the bottles were more often than not, made by witches, for want of a better word, or at least a person wishing ill on another, not as a means of killing or warding off witches.
Glueman mentions the habit of walling up a dead cat, which was to protect against infestation by rats or mice. Anyone wanting to see an example of this should visit the Museum of British Folklore caravan as I have one on loan from the Museum of Witchcraft in Boscastle.

www.museumofwitchcraft.com