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Toothpick Assays

 

This unorthodox procedure is being offered to demonstrate how with very little equipment and practically no expertise any prospector can examine potential ores and obtain valuable clues.

This type of assaying procedure is only a tool to see if a questionable mineral contains reducible metal from an acid solution.

Every prospector knows that the 1st order of business is to locate a mineral deposit from which samples are excavated. The tools for this job are the simple hammer, chisel & pick, to liberate specimens from surrounding rock. Likewise, the toothpick assay is just as uncomplicated. The difference is instead of using muscle and iron tools only a small flame and liquids are utilized to pry apart the bonds holding the mineral matrixes together and bring them back into unison as reduced metal.

Necessary supplies include:

  1. ceramic mortar & pestle for crushing pea sized rocks.
  2. nitric and hydrochloric acids.
  3. distilled water
  4. microscope.
  5. glass slide(s) to contain the powdered mineral and acid(s).
  6. tooth picks
  7. cigarette lighter
  8. 3 glass or polypropylene pipettes or standard eye dropper.
  9. eye glasses, and rubber gloves.
  10. ceramic or glass dish (concentrating and acid use).

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A thoroughly pre-cleaned Ceramic Mortar & Pestle avoids introducing contaminates thus assuring the prospector that what is seen through the lens is from the rock and not unintentional salting.

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The Coors ceramic mortar & Pestle at left is about 6” ID and about 4” deep. Depending upon the size of rock fragment and what my intentions are I use various sized mortars.

Time and experience has taught me to refuse using the iron mortar/pestle because it will invariably create iron shavings, which to an untrained eye under the microscope lens will cause confusion. When the novice peers through the lens and sees metal fragments it is easily assumed that that the pulverized minerals liberated these iddy-bitty particles, which unstintingly translates into a glory hole because these tiny iron metal fragments are often brilliant yellow with some hues of blue & red. If the iron mortar/pestle has to be used remove the metal particles with a magnet before proceeding with digestions.

Iron will often camouflage toothpick assay results, so why contaminate the proceedure? Sometimes, depending upon the nature of the sample to be tested I use a HCl pre-leach to remove most of the iron.

My favorite acid is HNO3 because it is great for dissolving many minerals/metals, especially silver and because it aids in the burning of the toothpick and makes finding Lead and Silver easy.

HCl acid is of no use for dissolving Ag, Hg, Pb & Tl. However, HCl can digest PbS (Lead sulphide or also known as galena), thus it has it uses in this form of testing.

Aqua Regia (3 parts HCL & 1 part HNO3) is often required to digest minerals & metals and most of the time I am able to use the toothpick assay to reduce some metals. When I cannot see metals and nitric or HCl did not produce any metal reduction on the tooth picks I then digest the sample in Aqua Regia to be sure that I am not over-looking potential goodies.

Distilled water is an absolute must when used in combination with acids. And I never use water from the spigot, because city water contains chlorine, and along with what ever else may be in tap water would or could be unintentionally introduce undesirable contaminates. Furthermore, any form of chlorine will cause potential problems with HNO3 digestions that could affect the end results. If tap water is used particularly for to larger concentrations of minerals such as panning then always rinse the concentrates 2 or 3 times with distilled water.

The microscope is an indispensable tool for the prospector and it is unimaginable that valuable tool is not part of the prospectors tool bag. It makes no difference what brand the microscope is, but it should be able to withstand rough handling and acid fumes. I prefer having several microscopes, but because the lack of money is a constant companion of the prospector a monocular has the advantage relating to cost. However, the monocular usually does not have a great depth of field for examining larger rocks.

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The Bausch & Lomb 5 Stereo Zoom (variable 10x to 40x) at left is excellent for rock samples up to fist sized, as well as glass slide studies. The monocular microscope (50, 70, 100 & 450x) at right is primarily for glass slides, but can be used for small rock chip samples. Many of the metals when reduced on a burning toothpick are difficult to determine if metal or not, thus another reason why it is necessary for variable magnifications. A microscope that will magnify in increments of 10, 20, 40, 60, 80 & 100 is ideal, with the emphasis being upon 20 to 60x.

Normal glass slides are just fine, but I prefer the slides that have a depression, well or hollowed out area to contain both acid and sample. Normal flat slides have a tendency, especially when heated to cause acids to migrate all over the surface. Yet, I use these all the time and these typical flat slides only cost about ¼ of what those with a well/depression do.

Now to the primary tool of this procedure -- the toothpick – made of wood and has a sharp point. Other wooden toothpicks are ok and sometimes are preferential. Of course the toothpick is useless without a flame source which will be used to ignite the toothpick tip that has been wetted with the acid solution that has digested a pulverized rock sample.

Any type of heat source is ok, but I don’t like having a constantly burning flame around during tests. Therefore the common cigarette lighter cannot be beat for simplicity.

Pipettes are what dispenses drops of water or acids and they come in all shapes and sizes, but for this particular exercise the polypropylene are best. These pipettes are usually graduated in milliters and thus a drop can be measured as a fraction of a ml for future reference & work. I use 3 of these pipettes – 1 each for water, nitric and hydrochloric acids. I use a magic marker to color the ends so I will always know what acid or water is being used.

Of course, eye protection and hand are important safety aspects of this whole adventure.

When a small sample is needed from a large mass of pulverized rock the gold pan comes in mighty handy. Similarly, when a rough concentrate is made often this heavier portion of the rock is then dried and pulverized to -100 to -200 mesh then re-concentrated in the gold pan.

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Finally, when doing these tooth pick assays I usually use a little ceramic or glass dishe for both digestions and as a mini-concentrator so I can better see the differences between the lighter and heavier portions of the pulverized rock in water under the lens.

Now to the application and results of the toothpick assay:

The following video presentations illustrate a magnified view (10x, 20x, & 40x) what occurs when a toothpick burns and metal is reduced.
Each toothpick tip has been dipped in a solution of silver nitrate.
A flame source (cigareet lighter ) ignights the wood tips, which continue burning without a flame.
The silver nitrate that has impregnated the wood becomes the heat source.
The very hot toothpick wood becomes charcoal and aids in the reduction of metal.
The burnt wood is replaced with strands of silver, which when heated again with a cigarett lighter flame melts and accumulates as small spheroids of silver.

6AgTP
5AgTP
2AgTP
1AgTP

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50x of tip of pointed wood toothpick that has not yet been wetted with water and/or acid. To the naked eye this tooth-pick appears to be sharp and not blunted as seen at this magnification.

Normally I try to wet the toothpick with as much pregnant (mineral laden) acid as possible.

Hydrochloric acid does not quickly soak into the wood very well and the toothpick tip will have to remain submerged a lot longer than either nitric or Aqua Regia acid solutions.

This next image demonstrates what the toothpick appears like after it was submerged in a solution of 1 drop nitric acid and 2 drops water that previously digested a small amount of silver metal on a glass slide.

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50x wetted/saturated toothpic tip with silver nitrate.
Obviously, the wood toothpick has swelled due to absorbing the nitric & water solution.
The color of the toothpick is very close to accurate.

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50x of ignited tooth pick with AgNO3 (silver nitrate) that produced this wire looking silver
The slightly yellowish coloration is due to reflections from the tooth-pick and shadowing.
The entire tooth pick that had been exposed to the silver nitrate has become very thin wires of silver.
This silver was reduced by the wood as the nitric acid and the wood burned from the cigarette lighter flame.
There was absolutely no effort in obtaining this reduced silver.
This ashed and now metal toothpick tip can be easily broken off on the glass slide so that further micro-chem testing can be applied to determine exactly what the metal is or is not. This type of assaying will be further explained under the title “micro-chem assaying”.

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70x of the above silver image.
The microscope mounted ccd camera cannot capture images in 3 D like the human eye does. Consequently, as the magnification increases the depth of field decreases, so I have to make compromises of clarity and over-all image quality in order to try showing what is seen through the lens.

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10x – The toothpick has soaked in a Platinum Chloride (PtCl) solution.

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10x – the above PtCl saturated toothpick was ignited leaving what appears to be simple ashed tip.
Normally I try to wet the toothpick with as much pregnant acid as possible.
Hydrochloric acid does not soak into the wood very well and the toothpick tip will have to remain submerged a lot longer than with nitric or Aqua Regia solutions.

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40x of the above PtCl ignited toothpick tip
This blackened, actually very dark gray is actually reduced platinum metal, but because the platinum is so thin and wire appearing it would not be recognized as metal at this magnification.

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50x of the above PtCl ignited toothpick tip.
This images begins to show how incredibly fine these reduced platinum metal strands actually are.

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70x of the same above image of the PtCl ignited toothpick tip
An incredible amount of extremely thin Pt metal wires, which have replaced the wood fibers.

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50x of the above PtCl ignited toothpick tip image, which was re-ignited and beginning to form a mass or clump of metal
Not 100% of the wood ignited sufficiently to get a good metal reduction of the entire wetted area of tooth-pick.

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70x of the above re-ignited PtCl image
Incredibly thin wires of platinum metal near the clump that have not yet fused with the clump.
Unfortunately, the cigarette lighter flame cannot get hot enough to actually melt this platinum into a bead/prill.
The next two images used the same procedure(s), which provided the reduced Rhodium metal.

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50x of burnt toothpick tip that was submerged in RhCl on glass slide.
Only the surface of the toothpick reduced this Rhodium Chloride to metal, because the toothpick was not saturated and the wood does not like to burn with only hydrochloric acid.

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70x of the RhCl on the burnt toothpick tip.
Re-focused on toothpick tip in an attempt to show more accumulated metal.
This and subsequent toothpick tips will be in different positions due to removing from under the lens and re-igniting or simply trying to find better areas to photograph.

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50x of Lead metal bead at tip of burnt toothpick.
A tiny piece of a reduced lead metal bead.
The toothpick tip was submerged in a solution of 2 drops water & 1 drop nitric acid that had dissolved a tiny piece of lead metal.
When a sufficient amount of the Lead was dissolved the toothpick was placed in the solution till thoroughly wetted and then ignited, which quickly produced this visible bead.
Sometimes, the lead nitrate is reduced to metal on/in the wood of toothpick so fast that as soon as the cigarette lighter flame touches the wetted wood tip it should be promptly placed under the lens to witness how fast the reaction occurs.

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50x of a small fragment of PbS (Lead sulphide) that was dissolved in 2 drops water and 1 drop nitric acid.
The toothpick was submerged in the pregnant solution and ignited.
Hundreds of tiny Lead metal beads are all over the burnt area of toothpick tip.
Some manipulation of the amount of water and nitric acid is often necessary to produce best results. Experimentation on known metals is probably the best way to learn this simple method of testing acidic metal/mineral solutions.

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50x of several Lead metal beads on tip of burnt toothpick tip that originated from a water nitric solution that digested some Native Lead.
There seems to be some crude on this toothpick tip along with the metal beads. I did not complete this test to see if the crude would also become reduced metal(s).

Keep in mind that this method of metal reduction can be and most likely is significantly contaminated with other metals. Nevertheless, the main focus of utilizing this type of testing is to provide quick and simple verification that a rock sample contains reducible metal(s).

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50x of a toothpick tip that was saturated in a gold chloride (AuCl) solution on a glass slide.
I could not focus on entire tip of toothpick due to the curved surfaces.
The primary reason for showing this image of this burnt toothpick tip is because it only looks shinny burnt wood. But, this burnt wood is actually partially burnt wood and oxidized gold.

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50x of the same toothpick as the above image that was saturated with AuCl, but was reignited.
At this stage there are hints of metal, but if I did not know this was only gold I would be tempted to throw the burnt tooth pick away.

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50x
The same above toothpick tip image that was again reignited to produce definite visible gold as extremely tiny thin filaments.
This image does not justice to the beautiful gold that the eye can see through the lens, because there are too many curved surfaces which cannot be focused upon simultaneously.

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70x of the preceding image.
Unfortunately this image does not adequately capture the beauty of these incredibly fine gold wires bundled together.

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70x of same gold tipped toothpick as shown immediately above, but was again ignited with the cigarette lighter flame tip to consolidate the gold.
The cigarette lighter’s flame does not get hot enough to consolidate this gold into a bead.