This herbal chemist on YouTube suggests using 60% vegetable glycerin, 40% water in a ratio of herbs of 8:1 (8 parts glycerin/water, 1 part herbs)...

However, he suggests that herbs without "volatile oils" need to be decocted with water first...

But herbs with volatile oils would only need to be added to the glycerin/water mixture without any prior preparation (the entire mixture then gets heated in a pot of boiling water for 6 hrs...).

Herbs with volatile oils and herbs without... I don't know what those would be.

Would milk thistle seeds be one of those herbs that does not have volatile oils?

Please, someone help me understand the chemistry behind this or how to make milk thistle extract with glycerin. I don't want to use alcohol to extract milk thistle. However, using glycerin seems to be a bit more involved...

Thanks in advance!