In general, the smoother and more rounded the piece is, the older it is, but that isn't always the case because it depends on what sort of bottom the lake has. Those with a lot of gravel and rocks will wear the glass quicker than lakes with a marl or soft bottom. The real thick glass is usually older because these days most bottles are much thinner than they used to be. Also, some of the pieces still have marks on them that help identify what they are. The marble I found for example was a bottle stopper from a Cobb bottle and they only existed in the late 1800s through the earlier 1900s, so we know it is about 100 years old. I found a large piece of green glass that I was able to identify through a couple marks on the bottom which told me it was 1 of 5 particular manufacturers and was from 1967.One piece of brown my wife found on the shore of Lake Huron had the letters FED on it. Through a bit of research I learned was part of the stamp, “FEDERAL LAW FORBIDS SALE OR REUSE OF THIS BOTTLE” , and was placed on alcoholic bottles from 1935 - 1964, so we know that is from a bottle at least 52 years old.The last time I was down to Wixom Lake fishing I found a few nice pieces of very thick, dark green. I have always been into history so this is right up my alley. We sure do enjoy the search and trying to figure out what we may have actually found. Of course a lot of it is simply guess work, but as you can see, some of it is a bit more specific.Some times you just never know. Just last week I found a prehistoric tooth in great condition, just lying there on the shore waiting for me to come along and find it.
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It is not about the equipment you have to use,
It is about how you use the equipment you have. :D