Optimized for gold?

[Anonymous]

HI all
I have always thought that there was very little difference between lead and gold as a detector target and many times I have used a piece of lead to test a detectors performance, however, a couple of recent episodes with a SD2100 fitted with a 10in Coiltek eliptical Mono and using the Coiltek 12v/7.3v battery system has altered my thinking on using lead as a test target.
I have recently returned from a detecting trip to the Central Vic goldfields and on two occasions I detected a tiny piece of gold and also a lead shot from the same mullock heap. On both occasions the gold was found first and produced a stronger signal than the lead wich was only found after a careful search. Detpth would in all cases be around the half inch mark.
The weights and dimensions of the gold and lead are:-
No 1 Gold 1.2 grains, size 2.5m/m Lead 3.5 grains, size 3.5m/m
No2 Gold 1.1 grains size 2m/m Lead 4.3 grains and 3.5m/m
It would certainly appear from these results that the Minelabs SD's are optimized for gold and that lead runs a poor second as a conductive target.
What are your thoughts???
Cheers
Steve D

 

[Anonymous]

The resistivity of pure gold is 2.05 * 10^-8 ohm meters at 273 degrees C. Lead is 19.2. So gold is almost 10 times more conductive than lead.
Probably neither the gold nor the lead shot were pure, so don't give too much weight to these numbers, but generally gold is a better conductor than lead.
For reference copper is 1.54 and aluminum is 2.42.
Robert

 

[Anonymous]

Hi Robert and Steve,
The relative conductivity of a metal as a percentage of that of pure copper is a little easier to visualise than resistivity or conductivity figures. If pure copper is 100%, then pure gold is 70% as conductive. Lead, as you say is much less conductive at only 8.4%. The problem comes when different metals are alloyed. When gold is alloyed with silver (105%) then its conductivity drops dramatically; even 5% silver makes a big difference. The same is true if gold is alloyed with copper. As I understand it, gold nuggets vary a lot in their composition and hence detectability. As an example of the variation in alloys, pure aluminium has a conductivity 61% that of pure copper. Aluminium alloy 220 is only 21% as conductive. I have never found any figures for gold alloys, but detection range tests that I did on gold/silver and gold/copper standards certainly demonstrated the drop in conductivity in gold alloys. How near it approaches lead, I do not know, but a detector that gives a good response on small lead objects will certainly have good sensitivity to gold nuggets.
Next week, I am expecting delivery of a Hocking Conductivity Meter, courtesy of Ebay. This has an inductive probe and displays conductivity as a percentage of that of pure copper. I will test some gold nuggets and other items, including lead, and report on the forum.
Eric.

 

[Anonymous]

Hello Steve, Eric and all,
I found lead to be very confusing as far as identification is concerned. I often find lead balls from shrapnell projectile (WW II era) and they give readings from 40 up to 80 (on my target ID VLF detector)even though they are the same as far as size and composition is concerned (I presume so, because it is on the same battlefield). What is more, the readings are quite consistent when a ball is dug up and lay on a soil surface - in the range of 40-45.
My guess is that lead procuces significant hallo, and the balls apear as more conductive targets when in soil. I thinkPI detectors are less suseptible to hallo, but anyway it is not a sample to test detectors in my opinion.
any comments on hallo in PI?
Piotr
PS
Eric, I think it is not a good idea to test conductivity of nuggets using eddy-current CM. They usally come with flat probes strongly (stronger than in target ID metal detctors) influenced by many factors as the size and lift-off. You can only compare electric properties (not copnductivity) of certain samples provided they are the same in size and shape. That's my oppinion.
I haven't even seen a real nugget, but I can test some lead balls using Foerster instrument.
What is the model of the Hocking meter?

 

[Anonymous]

Eric
Here are some numbers for copper-gold and silver-gold alloys.
The first column is the % of gold in the alloy.
The second column is the conductivity relative to copper of the copper-gold alloy.
The third column is the conductivity relative to copper of the silver-gold alloy.
% au cu ag
100 .75 .75
90 .14 .23
80 ??? .17
70 .11 .14
60 .12 .14
50 .14 .15
40 .17 .17
30 .22 .21
20 .30 ???
10 .47 .44
0 1.0 1.05
Lead shot is usually alloyed with antimony, so its conductivity is less than pure lead.
Robert

 

[Anonymous]

Hi Piotr,
The conductivity meter is the Autosigma 2000. I got it very cheap, and it will be interesting to play with. Never used one before and it may only have limited usefulness.
Eric.