Good coin bad coin? (Goldquest SS)

[Anonymous]

With the depth that I am now retrieving targets with the Goldquest, I am starting to unearth a lot of old coins in the water.
The problem that I am experiencing is that many of these coins are completely black in colour. Some of these coins are signalling off as "bad" targets, typical of iron objects, and not that nice coin/ring audio signature. The Australian $2 coin is the worst culprit. The metal composition must be playing a part in all of this. Our $1 and $2 coins are gold coloured and I guess have a higher copper content compared to all the other lower denomination cupro-nickel coins.
I can only assume that the black coating is now what my detector is "seeing". The coins that are green and encrusted still signal like a nice target. These black coins have no encrustation, they are just completely coated black. If you rub hard, some will come off on your fingers.
If anyone can throw any light on this matter or they have experienced something similar, I would be glad to hear from them. <img src="/metal/html/confused.gif" border=0 width=15 height=22 alt=":?">
Generally, the audio from the Goldquest SS is telling the operator far more than they probably realise. Iron can be identified on a regular basis, and not just from double blips. It peaks extremely quickly, and hasn't got that satisfying sound that coins/rings usually give.
Tony.

 

[Anonymous]

Hi Tony,
Have you tested the coin with a magnet, to make sure there isn't a ferrous core? Some of our coins are now plated steel, although they are lower value coins. From my observations, any coins that contain copper usually go green or red in sea water.
Eric.

 

[Anonymous]

G'day Tony, The black discolouration on the "gold" coins is probably due to aluminium corrosion. According to my Renniks coin book our 5/10/20/50cent "silver" coins are 75%copper/25% nickel; and the $1 and $2 coins are 92% copper,6% aluminium,2% nickel.
Hope that helps explain things.
regards, Allan in Gympie.

 

[Anonymous]

Allan,
Thanks for your diagnosis !!
I never realised there was an aluminium content to our $1 and $2 coins.
I guess it's fair to assume that this may be the cause of the sometimes poor signal.
Do you detect in Queensland waters or is too dangerous!! <img src="/metal/html/shocked.gif" border=0 width=15 height=15 alt=":shock"> <img src="/metal/html/shocked.gif" border=0 width=15 height=15 alt=":shock"> <img src="/metal/html/shocked.gif" border=0 width=15 height=15 alt=":shock">

 

[Anonymous]

Eric,
Received a post from Allan who has informed me that those particular coins of ours have a 6% Aluminium content.
All Australian coins are the same metal all the way through (no clads or cores).
Would you agree that a heavy coating like this can mask the true metal underneath? My understanding is that the eddy currents form on the surface of the metal, and there is no actual penetration into the metal.
Learn something new every time I go out detecting <img src="/metal/html/glasses.gif" border=0 width=15 height=15 alt=""> .
Regards
Tony.

 

[Anonymous]

Hi Tony,
Eddy currents are often thought of as a skin effect phenomena, as a spin off term from r.f. engineering. The penetration of an external magnetic field into a metal object depends on the frequency of the field. High frequencies only have a small penetration, while low frequencies can penetrate deeply into an object. Regarding our particular problem, the penetration of the transmitter field into a coin depends on the time that the field is switched on and the conductivity of the metal. When running the Goldquest at minimum delay and highest pulse frequency, the transmitter field is only switched on for 30uS. This is fine for small gold jewellery, the majority of rings, and medium/small cupro-nickel coins, but for larger and more conductive coins it is too short. For copper coins and silver coins you may have to increase the TX pulse width to around 300

 

[Anonymous]

Eric,
Thanks again for the information.
The features and strengths of the Goldquest that you describe is precisely why I purchased the unit.
Digging coins doesn't excite me much, but it covers the cost of a few cold beers at the end of the day. <img src="/metal/html/biggrin.gif" border=0 width=15 height=15 alt="">
People are amazed when I show them the detection depth of a small gold ring against that of a larger coin. The difference is astounding with the gold ring having a considerably stronger signal for the same height/depth below the coil.
Tony.

 

[Anonymous]

Tony, Haven't done any beach detecting yet. Will try it out when I get my PI-1 going and waterproof. Southern Queensland isn't too bad for beasties like the stringers they get further up north. But I wouldn't like to walk on a bullrout or a stonefish.They just lay there like a deathadder waiting to be trodden on.Probably unlikely on a busy surf beach.
regards,Allan in Gympie.

 

[Anonymous]

I have found quite a lot of one and two dollar coins at the beach but in the dry and wet sand and have never seen the black colour that Tony described until today. I hunted a saltwater lake that has been recently drained, so much of what I found had been underwater along time and up popped the black 1 & 2 dollar coins. The other denominations were black or brown too but thats more usual. I now have to admit I was not using my GQ as the number of twist tops etc. was driving me nuts so I resorted to my Excalibur but had to turn the sensitivity down because the large amounts of black sand present. Interestingly the Excal ID them as 1&2 dollar as usual. I suppose this is due to the lower freq.and better penitration of the coin surface as decribed by Eric.I have posted a photo below showing the found coins on the top row and normal coins on the bottom row.
Bill.

 

[Anonymous]

Bill,
I'm glad someone else is digging those mysterious black $1 and $2 coins. The Goldquest still picks these up deep. Mine are very black and smooth, with no encrustation. All the other old coins are green, black, or red.
I'm certainly glad the government got rid of the 1c and 2c coins altogether (although they do turn up occasionally), and then ditched the $1 and $2 notes for coins.....I hear the $5 note might be on the way out in exchange for a $5 coin !!!
At least the government is good for something <img src="/metal/html/lol.gif" border=0 width=15 height=15 alt=":lol">
Tony.

 

[Anonymous]

Tony
On the beach this morning,sea was rough so I stuck to the wet sand well away from waves and found a few encrusted dollars etc.and a large brass bangle. Surprising few targets on the wet sand,I think the big seas my have deposited more sand than it looked. I hope that $5 coin comes along soon.
Bill. <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt="">

 

[Anonymous]

Or may have stripped a lot of this sand and dumped it all in the water, along with all the targets?
It happened recently over here. There is a lot of sand in the water at the moment and it needs to be moved !
Tony. <img src="/metal/html/angry.gif" border=0 width=15 height=15 alt=":mad">