Further Resources
The Langridge website has the safety data sheets for their products, and recipes using their materials:
Langridgecolours.com/resources
I highly recommend this book by Tom Mazzullo and Susan Schwalb for a comprehensive explanation of metalpoint:
Silverpoint and Metalpoint Drawing A Complete Guide to the Medium
Koo Schadler is another expert in the medium (as well as egg tempera), her website has information on silverpoint techniques available to download www.kooschadler.com
The MITRA (Materials Information and Technical Resources for Artist) forums are invaluable for up to date information. Search silverpoint and/or metalpoint to see what people have already discussed www.artcons.udel.edu/mitra/forums