As
DeeAnna points out, you would be wasting the Arnica by adding it to lye soap. It is quite nice in a balm. I buy mine at HEB in Texas when visiting in that area. One jar lasts me a long time, and it's fairly inexpensive. Walgreen's carries it as well, so I don't really have to go to Texas for it, however, I visit family regularly & did live there for a brief while.
If you want long lasting effects of Arnica and want to make your own products for your Husband's pain, I'd suggest trying the balm first and see if it helps him. THEN, if it does, you can buy the extract and incorporate into a home-made balm.
Here is a
link to the powdered extract, which you can purchase (or find elsewhere.)
There are
contraindications to working with or using arnica: If YOU are pregnant, or breast feeding you should not work with or use arnica. If you or DH is allergic to ragweed, marigolds or sunflowers, neither should work with or use it. If taking warfarin or other blood thinners, arnica should not be used as it can potentiate the effect of blood thinners. Also if taking aspirin or other platelet inhibitor drugs, arnica should not be used as it is also a platelet inhibitor. It can cause contact dermatitis, so while handling raw, wear gloves.