PimentoUK

Indeed, the tin mines were in Cornwall , the most South-West point of England, and the tunnels went out under the Atlantic:

Here's an article, sorry it's the Daily Mail, they spam adverts and trash all over the page:

https://www.dailymail.co.uk/news/article-2285245/Cornwall-tin-mines-How-quest-metal-took-men-just-feet-ocean-floor.html

And one of the mines is now a tourist attraction:

https://www.visitcornwall.com/things-to-do/attractions/west-cornwall/lands-end/geevor-tin-mine

Steven : It's electrical conductivity ... how good would it work if you made an electric cable out of it ?

Copper, silver = great choices

bronze = not that good

lead, tin = going to need pretty thick wire

cupro-nickel, stainless steel = they will be glowing red all the time

Here's a decent reference table for many 'industrial' metals/alloys:

http://eddy-current.com/conductivity-of-metals-sorted-by-resistivity/

Yes, we had good deposits of iron ore, lead and tin. The lead also contained silver, as a bonus. The Romans were pretty skilled at metallurgy; bronze, pewter, solder were all tin-containing metals.

https://www.romanobritain.org/11_work/raw_mining.php

There is native gold in the U.K , there are deposits running down the west coast of Scotland, through to the central mountains of Wales. Some was extracted from the rivers, and in Roman times, there was some mining in Wales.
But most of our gold was imported from the continent.

Roman mine in Wales:
https://en.wikipedia.org/wiki/Dolaucothi_Gold_Mines

Some background reading on gold artefacts in the UK from Roman to Medieval times:

https://finds.org.uk/counties/blog/gold-working-in-the-roman-and-medieval-periods/

Steve: The 'Source' function has gone again, as have the 'subscript' / 'superscript' options.
I've just tried to type some maths in one of my posts, and stalled when I couldn't find how to type 'Q - squared' , even after shrinking down the screen size twice.
Any thoughts?

It's funny nomenclature, but that's what it is. It may be how they were known in Medieval times, ie. a silver penny and a gold penny ( worth 20 silver pennies ) . They did keep detailed written records back then of coin production, how many they made, how many were withdrawn, who funded the production, who was the mint manager responsible etc etc., so I'm sure there's some reference to "the stryking of ten thousand golde pennys" in some old ledger in the Royal Mint's archives.

The term 'penny' goes back a long long way, it's from the Germanic 'pfennig', from 500 AD or earlier, and we Brits adopted it along with their coins in the 600's. It's meaning was essentially 'coin' or 'money' , the concept of 'twopenny coin' or 'sixpence coin' didn't exist. Later coins, in the 800's, were cut in half or in 4 to make halfpennies and fourthings ( modern 'farthing' ), but it's rather blatant that half a coin is half a penny, no-one is going to be confused or tricked.

Quote:

"The -3db point yields max returned signal of a given center frequency"

No it doesn't. It's just the reactive element that is greatest. Using my prior example of the F75 ( 13 kHz machine ) detecting the silver Dollar: The coin in fact gives a very strong signal to the detector, as you would expect a large lump of metal to do. It's just that the phase lag is very much towards the '90 degree' region, as the detector freq is massively higher than the target freq. So it's not hard to detect, if you look at the correct signal(s).
What affects the targets 'detectability' is the processing the machine uses to determine what's there. The F75 has two different modes: 'DE' = default, and 'JE' = jewelry. Fisher don't spill any beans about what the difference is, of course. My hunch is the JE mode does full vector addition of the I and Q signals to determine target strength,

ie. (signal = I2 + Q2) , whereas DE mode does a linear addition, (a I + b Q ) , which is probably similar to what classic analogue machines do.
So you can now see that if you have multiple frequencies ( ie. 2 or more )  to interrogate the target & ground, you have plenty of ways of analysing each freq and combining them in a way that produces a target/no target outcome.

As you have a specific interest in Minelab's 'Multi- IQ' techniques, it would be useful for you to understand some basics of what the machine actually does.
Cut 'n' pasted from other posts of mine:

Both the 600/800 models use 7.8kHz, 18.2kHz and 39 kHz in Multi-IQ .
Beach modes have an additional 13kHz in the mix.

The detector transmits a complex square-edged waveform, that repeats every 385 microseconds. In that waveform are 15 cycles of 39kHz, 7 cycles of 18.2kHz and 3 cycles of 7.8kHz. That is how the operating freqs are related to each other : 7.8k : 18.2k : 39k are ratios of 3 : 7 : 15.
Beach modes ( and apparently all Vanquish's ) use 4 frequencies, 7.8kHz, 13kHz, 18.2kHz, 39kHz in a 3 : 5 : 7 : 15 ratio.

The single-frequency options do exactly what is claimed, they are 5 / 10 / 15 / 20 / 40kHz.

My personal opinion is that:
Park1 and Field1 place the emphasis on the 7.8 kHz, and use the other two freqs to help reduce ground signal.
Park2 and Field2 place the emphasis on the 18.2 kHz, and use the other two freqs to help reduce ground signal.
Gold1/2 place the emphasis on the 18.2 kHz and 39 kHz , using the 7.8 kHz to reduce ground signal. (I have a Eqx600, so have no experience of what the Gold modes do differently )

In addition, I speculate that the 'intelligent' technique ML are using is this:
Example: In Park1 , they are also still analysing like Park2 and Gold2, as a 'background' process. If a target gives sufficient response at these medium / high weighted freqs, it will trigger an audio response. So you would still get 'hits' from some small low-ID targets, that would otherwise have been ignored if the machine had been running as a 'pure' 7.8 kHz ground-compensated machine.
Ditto, gold modes would likely 'hit' some larger/high-conductor targets that 39 kHz on its own wouldn't be so hot on.

As for the 'Beach modes':
It's known the transmitted freq mix is 7.8 kHz / 13 kHz / 18.2 kHz / 39 kHz for both modes.
The processing of these signals is going to be firmly aimed at reducing the salt-water dependant ground signal.

The official record of the find:

https://finds.org.uk/database/artefacts/record/id/1039261

..and a 'just dug' shot, that was posted to Facebook:

Getting complicated, now.

Detectors don't just measure the reactive component of a target. If they did, a 13kHz machine ( decent all-rounder Fisher F75, for example ) would be hopeless at finding 1 kHz targets ( US silver dollars / half-dollars ), as the phase lag is about 86 degrees, with the reactive component way less ( 7% ) than the resistive component.

"I would tend to think this has a lot to do with Multi-IQ design and resulting algorithms"

Now you're bringing multiple freqs into it ... and it sounds like you don't really understand the why's of multi-freq.

Using multiple frequencies has very little to do with "hitting the target with a range of freqs hoping that one will hit the spot". It's primarily about working out the ground signal, so it can be largely eliminated, thus making the target more visible.

The target can be modelled as a simple first-order low-pass filter, but rather than the familiar R-C components, it's an L-R circuit. The -3 dB freq is the freq at which the phase lag is 45 degrees.
If you measure the target at another freq, you will get a different phase lag, which can be measured. Mathematics will then allow you to calculate the target's -3 dB frequency.
However .... targets can have and do have frequency-sensitive characteristics. This is due to induced circulating currents not penetrating deeply into the target ( skin effect ). As a result, bulky, thick targets will behave slightly different at different freqs. Low test freqs will see more of the skin than a high test freq. So to get the best target freq measurement, you should use a test freq close to that of the target, maybe +/- 30%. Measuring a 1kHz target at 18 kHz is likely to include some target-dependant error.

Probably the simplest way of measuring a target's frequency is to use a good target ID detector, which you have calibrated the scale. The Fisher F75 is pretty good for this, having a large non-ferrous range. All you then do is wave the target over the coil, read the ID, and look up on your calibration chart/table to see the -3 dB freq.

Also: determining what is the best detector freq to use for a given buried target is another more complex problem. Detectors also measure the ground: a higher detector freq give stronger ground pickup, which compromises its ability to distinguish the target from the ground.
So that's one plausible reason why a detector operating below the desired target freq is best. But there are arguments going the other way ( I forget what ... ) suggesting a higher detector freq is optimal. In practice, it doesn't seem to be too critical, thankfully.

You can manufacture dummy targets from loops of wire. By calculating the loop resistance, R, and loop inductance, L, you can calculate the -3 dB freq

w = 2 * pi * f = R / L

As a result of some target-modelling tinkering we did on the Geotech1 forum, I have a dozen or so of these targets, made from copper wire. Theory and practice agree well, for PI and VLF machines.

Our big bronze predecimal penny ( 1860 - 1967 ) reads TID=80 on the Fisher F75 when fresh, sort of where you would think it belongs. But 50+ years of rot, and they're typically reading 55 - 65 TID. I have dug a couple that had ID pretty much right, one was a last issue '67 in a park that may have had fresh topsoil added, so probably lacking in typical farmland fertilizers/animal excrement/urine chemicals.

 Curiously, our cupro-nickel coins seem little-affected by corrosion. They look awful, and clearly have been pickled, but the TID drop is minimal. Maybe it's because CuNi is a lousy conductor to start with, so can't get much worse? Our oldest CuNi coins are 1947, they're mostly near-identical alloy to your US 5c coin.

US coinage is not my field, but the metal compositions are definitely not correct in that table.
Wikipedia has a summary of their alloys:
https://en.wikipedia.org/wiki/Penny_(United_States_coin)

It would be educational if you tested a circulation Indian head coin ( not a dug-up ), to give a baseline figure for how high they can read ( conductivity ) , then the drop in reading due to corrosion becomes clearer.
We have similar bronze coins here in the UK, and the drop in Co value due to corrosion is marked, and very variable.

Based on a post Dilek made over on Tom Dankowski's forum, NokMak intentionally chose to keep the Legend straightforwards.
She stated that they listened to what their customers wanted ( in particular related to the naming contest, where entrants thoughts/wishlists were requested ), and designed the machine so it satisfied the most popular wants.
This has other advantages for them:
* It makes designing/producing their first smf machine simpler and quicker.
* Their machine is easier to use for beginners or less experienced detectorists.
* It's easier for them to subsequently produce a better machine, with more modes/options/features/bells&whistles.

Both the Eqx and the Legend are missing plenty of features. True all-metal mode; non-motion modes; stereo-audio operation; nice to have them, but not that popular with the majority of users.
Minelab also have a trick they could 'pull out their sleeve', of course. Make the Eqx behave like a CTX3030/E-Trac. The standard Eqx coil is capable of 5kHz to 40 kHz operation, so some slightly-modified version of the 3.125kHz/25kHz FBS waveform could be achievable, eg. 4.75kHz/38kHz, it may work OK?

"Nokta Makro’s simultaneous multi frequency competitors provide a weighted range of frequencies utilized in their respective detection modes/programs"

But they don't.

The 'weighting' affects how they process the frequencies, not what frequencies they use.

The Equinox simultaneously transmits 7.8 kHz, 18.2 kHz and 39 kHz in Park1/2 , Field1/2 , Gold1/2 modes, this is an easily observable fact.
My personal opinion is that:
Park1 and Field1 place the emphasis on the 7.8 kHz, and use the other two freqs to help reduce ground signal.
Park2 and Field2 place the emphasis on the 18.2 kHz, and use the other two freqs to help reduce ground signal.
Gold1/2 place the emphasis on the 18.2 kHz and 39 kHz , using the 7.8 kHz to reduce ground signal. The difference between the two gold modes is something I could only speculate about.

In addition, I speculate that the 'intelligent' technique ML are using is this:
Example: In Park1 , they are also still analysing like Park2 and Gold2, as a 'background' process. If a target gives sufficient response at these medium / high weighted freqs, it will trigger an audio response. So you would still get 'hits' from some small low-ID targets, that would otherwise have been ignored if the machine had been running as a 'pure' 7.8 kHz ground-compensated machine.
Ditto, gold modes would likely 'hit' some larger/high-conductor targets that 39 kHz on its own wouldn't be so hot on.

As for the 'Beach modes':
It's known the transmitted freq mix is 7.8 kHz / 13 kHz / 18.2 kHz / 39 kHz for both modes. So in addition to being enhanced by a fourth freq, the relative levels of the other three will be changed, too.
But the processing of these signals is going to be firmly aimed at reducing the salt-water dependant ground signal, and the opportunity to 'pluck out tiny gold earrings whilst searching for silver coins' etc is lessened.
I've not yet taken my Eqx to the beach, and have only briefly played around with Beach modes on land, so I have no insight into what is special/different about them.

To further complicate matters, the Eqx is software upgradable, so it's possible, though not likely, the transmitted frequencies and / or the processing techniques could be modified at some point. This would also apply to NokMak's Legend, I assume.

You're getting 'concerned' about something that's unknown. There are no Legend's in the hands of folks who can put a spectrum analyser on it and get some insight into what it does/doesn't do, what frequencies it uses in what modes, etc. The Equinox is less unknown, but even then the exact algorithms/techniques used are hidden in software, not disclosed by Minelab, may be patented, or may be just proprietry knowledge/trade secret.

Regardless, it's not going to make any comparisons easy.

And these are just the first models of both companies smf range. Clearly there's loads of extra features could be put onto high-end Equinox successors, and Nok-Mak are certain to be thinking ahead to other new models, and will be studying the feedback from Legend users ... just a Minelab will have done with the Equinox user reports.

For you US folks, there's another alternative to Acid Magic:
"Acid Blue" by Champion:

https://www.leisurepoolinc.com/shop/pool-spa-chemicals/balancers/ph-balancers/acid-blue-buffered-low-fume-muriatic-acid-1-gallon/

I believe it's "Certol International Acid Magic" , aimed at swimming pool cleaning:
https://www.certol.com/landingpages/acidmagichome/

The safety data sheets give little insight into it's chemical composition, other than hydrochloric acid plus 'trade  secret mystery ingredients', which are certainly what chemists call 'buffers'.

Despite their 'International' name, their products seem limited to US and Canada. So no chance of finding it here in the U.K.

( domestic pools are not common, here, either, so finding an equivalent could be tricky)

Erik : There is hardly any difference between a 17" coil and a 15" coil ( which is commercially available ), so your new request has little purpose. Stick with your original wish.

Sergei in NY has a website with some lengthy cleaning guides. There's plenty of choices, that's for sure.

https://www.metaldetectingworld.com/cleaning_preservation_coin.shtml

There's also an iron electrolysis guide, as I know some of you US guys dig iron relics:

http://www.metaldetectingworld.com/electrolysis_rust_removal.shtml

Another iron electrolysis guide, 'Andy's Patch':

https://qsl.net/2e0waw/rust.htm

-------

I've just read the 'Western & Eastern' article, and their opinion about tombac buttons:

The only way to clean these up is mechanical cleaning, any acid etc technique will destroy them.

I found that extremely fine grade abrasive paper, like silicon carbide, wet'n'dry, will remove dark stains, green deposits etc, whilst only slowly attacking the grey patina. Don't use anything coarser than 800 grade. Use 1000 or 1200 grade, and finish off with 1200, followed by rubbing with plain white paper and a buffing on an old cotton rag.

( '1200 grade'  means 1200 grains-per-inch, possibly in different countries there are different labelling systems ).

As us European detectorists dig up a wide variety of stuff, from 3000 BC to present-day, made of all manner of metals, I suggest registering with one of the better-known UK forums 'MDF' . That will give you access to a wide selection of discussions about all kinds of cleaning/preservation methods.
The 'cleaning finds' sub-forum:
https://www.metaldetectingforum.co.uk/viewforum.php?f=122

As you specifically asked about copper-alloy items, one of my own 'discoveries' for cleaning bronze items involves stiff bristle brushes.
I'll re-post here:

I've discovered a way of cleaning up bronze coins/artefacts with pleasing results, that I've not seen mentioned anywhere else, so I thought I would share it.

The patina on bronze items is often quite fragile, and you can see time and again forum posts related to this. Many mentions of washing a coin and losing all the detail, the patina going dull and rough. Comments from experienced folks saying they never get finds wet, and use minimal cleaning, such as wiping the bulk of the soil off, leaving it in the detail of the coin.

Despite the fact that the item has been in wet soil for centuries, I too think that fully drying it, then cleaning it, is the way to proceed. And with the right tools, the results can be very good.

Amongst my box of random tools, I have some well-worn artists stencil brushes. These have a lot of densely-packed short stiff bristles, usually hog bristle. I used one to brush off the dried dirt from a coin, and it was very effective, but it didn't damage the underlying patina. After more brushing, the patina started to take on a noticeable shine, as it was obviously being slightly abraded by the rough bristles. When I'd finished, the coin had an attractive darkened shiny look, and a gentle rub on a cotton cloth ( ie. my t-shirt ) finished it off. It wasn't as glossy as a waxed item, and of course no oils/waxes are involved, so it's better from a preservation viewpoint.

I've attached a picture of my brush, the bristles are very short, about 4mm , and the brush diameter is 8mm. I've had a scour of the internet, and similar brushes are available on eBay and Amazon (and your local art supply shop may have them too ).
Search for:
stencil brush hog
stencil brush bristle
stencil brush Jakar

Many of these brushes are huge, you need to find the small 8mm or "Size 1" ones. Priced around £3.00.

They have longer bristles than my worn ones. Try them as they come, but consider trimming them with a very sharp knife to get the stiffness. They will eventually wear short, but I've successfully trimmed off 3mm from the end of the aluminium ferrule with a Junior hacksaw to give new life to one of mine, so you should be able to get good value out of one.
Do the brushing in a well-ventilated area, as clouds of soil dust are generated.

Attached photo of a pretty Tudor hooked fastener that's been cleaned this way. There's a few harder deposits in the details that the brush didn't move, I need to carefuly have a go at them with a pointy tool, then re-brush them.

To summarise:
if you've dug a bronze item that may have a good patina, remove the loose dirt, wrap it in tissue and put it in your 'finds pot'. Let it dry out for several days at home so the patina is hard, then clean and shine it with a stencil brush.

The cynic in me thinks that is a photo of empty cartons that have just been delivered to the factory. Hence why they've been opened up. Notice the flimsy single layer of parcel tape that's been cut through to open the boxes?