Platdigger said:
Because you have mixed them from so many different years of manufacture, I doubt it will be easy to get a truely representative assay of the entire lot.
Platdigger is right. The value spread in those pins could cover the entire gamut from maybe .25 g/# to 3 g/#. The overall accuracy of the assay is very dependent on first getting a representative sample. In sampling most anything, getting a 100% representative sample is impossible, unless you run the entire lot. However, there are sampling methods that could greatly improve the end results.
There are lots of options. For those mixed pins, I can think of about 3 ways that you could do it. In any case, tumbling or blending would probably aggravate the situation. I think the entire batch would segregate and the smaller pins would tend to end up on the bottom.
(1) The simplest, but most probably the least accurate method, would be to first spread them out, then take small scoops from here and there and combine them. The problem with this is obvious - where do you take the parts from?
(2) Another way would be to use a sample splitter. Here is what one type looks like. As you can see, they are expensive.
http://www.lmine.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=riffle
You simply pour the pins in from the top using a dustpan. The riffles separate the material into two portions. On the two sides, there are pans that the material falls into. Every other riffle goes to one pan and the other riffles go to the other pan. You re-split the material from one of the pans. This is continued until you end up with about one pound. If you start with 150 pounds, this would take 7 splittings. However, as you can see, these splitters are available with different sized openings. I would be hard pressed to advise the best size for these pins. The size could make a difference in the accuracy.
(3) Coning and Quartering. This is an ancient sampling method that you can do without buying any equipment. For the pins, I think this would be as good (or, maybe, better) as using a sample splitter. It is much more laborious, however. I could probably go through the whole cone and quartering procedure on the 150# of pins in 30 minutes or less.
(A) On a clean surface (floor), all the pins are slowly poured or added with a scoop, from slightly above, onto a
single point on the floor. As you add the pins, they will form a cone shaped pile. The pins are always poured or added on the apex of the cone as much as possible. As the pins are added, they will slide down the cone shaped pile somewhat. They should slide down equally all around the cone. This is very easy to do but hard to explain. In order to confine the parts added to the apex of the cone, some people fix a stationary funnel or hopper and add the parts through that. With a funnel, I would select one with about a 2" opening, in order that the parts run free through the hole and don't jam.
(B) Flatten out the pile to about 1/2"-1" thick. A fairly large piece of thin sheet metal will work. Push straight down and wiggle the sheet metal slightly as you work. The resulting pile should be fairly circular and of uniform thickness.
(C) Divide the pile into 4 fairly equal parts.
See the attachment. This can be done with a piece of sheet metal.
(D) As explained in the attachment, remove 2 opposite segments. The other 2 segments are combined and the coning and quartering is repeated. This is continued until you have about 1 pound of pins - 7 times for 150#.
In any sampling method, you try to take yourself out of the picture. For example, it would be unwise to try and estimate the mix and then select quantities of specific parts to satisfy your estimate.
For much greater accuracy, more than one sample should be run. The results are then averaged. For each sample, the entire cone and quarter process should be repeated.
In fire assaying, when lots of copper is involved, the sample size is limited to just a few grams. This problem can be eliminated by first dissolving the copper from the 1# sample(s) in nitric acid or HCl/H2O2. All the residue from each sample is collected on a separate filter paper, rinsed well and dried. Then, I could fire assay the total sample(s) residues, paper and all. The nitric prep work could be done by you or me. If I did it, I would have to charge for it.
It is also possible to melt (the melt must be stirred well) the pound sample of pins and then drill the bar or button in several places and send me about 5 grams of drillings. The problem is that some of the copper will oxidize (try to melt with reducing conditions) and the assay results will be high. Weigh the bar or button before drilling. That way, if you have exact weights of the pins being melted and of the bar or button, a compensation can be made in the final calculations. This would assume, of course, that no gold is trapped in the copper oxide. With a good melt, it shouldn't be.
Note that this specific info is only good for that particular batch of pins. Every deal is, at least, a little bit different. Good sampling is an art/science unto itself.
Good sampling is extremely important but it ain't that easy to do.
Chris Owen
