Originally posted by Skippy
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I did suggest using copper (pre-1982-ish) US 1 cents for several reasons. One was for consistency, if someone else is to reproduce a test, they don't want to do so with a random mix of zinc-cored and copper coins. Secondly, the copper coins are the close match to our U.K bronze 1 pence coin (we went to a copper-plated-steel coin in the 80's, for the same economic reasons as the US went to zinc-core). Also, I understand 'zinc' cents corrode badly in the ground, this makes them less suitable as a test target, im my opinion.
Re: the lead disc... the size/weight I've started off with (60 x 5, 160gm) was an engineering guess, based on a number of ideas. Clearly it's significantly larger than any coin-size object, so that puts in in the range where it should be detectable deeper than any coin. If you're searching in a way that reduces pickup of small coin-size items (eg. coil clear of the ground), then it will still be visible. During my own real-world deep-searching trials, I did detect a golf-ball sized lump of lead weighing 150 grams... it was only about 10 inches (30cm) down, so would probably have been detected with normal search methods. I had the search-coil about 40cm above the ground when searching.
It may well turn out to be a bit too large and easy to detect, but that's the point of experimentation, you've got to start somewhere.
I like the lead disc as a target because it could be representative of a real target - some coin hoards have been found buried in lead pots, which are often fragmented. Plus it could also be oriented vertically or at an angle to give that 'coin-on-edge' effect (see Riss' first post).
Regarding my mould for the lead: My first attempt was made from thick aluminium foil (actually 0.07mm) as used for meat pies, pastries etc by food retailers. I found some jam/preserve which had a lid about the correct size, and shaped the foil around it, creasing up the edge. It was about 12mm deep and 58mm diameter when I'd finished, close enough. The correct weight of clean-ish lead was weighed out before melting, allowing 5 grams extra for losses, eg. dross, metal staying in the melting pot etc. The mould was placed on a ceramic tile for thermal reasons.
I suppose I should state: take precautions. Gloves, eye protection, well-ventilated area, water-soaked cloth to hand. etc etc.
Re: the lead disc... the size/weight I've started off with (60 x 5, 160gm) was an engineering guess, based on a number of ideas. Clearly it's significantly larger than any coin-size object, so that puts in in the range where it should be detectable deeper than any coin. If you're searching in a way that reduces pickup of small coin-size items (eg. coil clear of the ground), then it will still be visible. During my own real-world deep-searching trials, I did detect a golf-ball sized lump of lead weighing 150 grams... it was only about 10 inches (30cm) down, so would probably have been detected with normal search methods. I had the search-coil about 40cm above the ground when searching.
It may well turn out to be a bit too large and easy to detect, but that's the point of experimentation, you've got to start somewhere.
I like the lead disc as a target because it could be representative of a real target - some coin hoards have been found buried in lead pots, which are often fragmented. Plus it could also be oriented vertically or at an angle to give that 'coin-on-edge' effect (see Riss' first post).
Regarding my mould for the lead: My first attempt was made from thick aluminium foil (actually 0.07mm) as used for meat pies, pastries etc by food retailers. I found some jam/preserve which had a lid about the correct size, and shaped the foil around it, creasing up the edge. It was about 12mm deep and 58mm diameter when I'd finished, close enough. The correct weight of clean-ish lead was weighed out before melting, allowing 5 grams extra for losses, eg. dross, metal staying in the melting pot etc. The mould was placed on a ceramic tile for thermal reasons.
I suppose I should state: take precautions. Gloves, eye protection, well-ventilated area, water-soaked cloth to hand. etc etc.
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