Just as with the two seminars I did the end of July, in all of them I try to use an assortment of coins I have on hand in my kit so they can see what the Target ID or VDI numeric readings are for desired coins. Mainly the Indian Head cents as well as many early "wheat-back' Lincolns from 1909 to about 1920, and even some during the '20s.
The early "fatty" Indian head and Flying Eagle cents will read much lower, but the common 'thin' or 'standard' size Indian heads from 1864 on through 1909 and most/many 'wheat-backs' to about 1920 and many up to 1930ish, will produce a Target ID (category or target name) or a VDI reading (numeric reference) that is similar to the modern Zinc cent or even lower into the Screw Cap range.
These targets might show some wear, but most of them look quite good, and these are "air sampled" comparisons so the ground mineral signal isn't a factor. I can use any detector attendees bring, and even those who are coin collectors and know the metal composition of these coins are listed as being the same are surprised at the low-readings from those older coins.
Why do they read low? The metal source can be the only answer, combined with the refining process to produce the copper. The source and refining of the copper is why we have to go after those lower-reading targets if we want to get the older coins (cents). Then, add the ground signal, nearby trash targets that can mask a desired find, or just having a bad coil, poor settings, as miserable day, whatever the reason, it all adds up to having to recover a LOT of targets that don't read 'normal'.
Monte