I find it interesting that people sometimes don't know certain things unless they experience them.
I'll write this one more time so that perhaps it can be understood a bit better:
In about 1975 I was attending Olympic College in Bremerton, WA taking pre-electrical engineering classes. There was some excavation being done there to remove soil that had at one
time been home to a large garbage dump. Accrding to the men working on the job the trash hadn't been found very deep, but it did cover a very large area. A few old bottles had been unearthed and laid on the dirt removed to dig a ditch. I was allowed permission to search the area with my metal detector. I unearthed a few artifacts and continued on with my search. No coins were found but I did unearth several old bottles, one of which was an ink bottle, and two in a clean area which produced a faint signal like a small piece of tinfoil at 4-6" deep. Both were a faint grey color. There was no other metal in the ground within 6' of the two bottles. Both were lieing side-by-side. Maybe someone realized somewhere along the way that they were unusual, I don't know, but as I said before, bottles which have a metalic substance in the gals can give a metal signal. Both were made before 1920 when glass was still seamless and both had been hand blown. Yes, I checked the bottles separately and away from the soil just to make sure, and yes they gave an audio too.
I hope this helps. And HH.
Larry