Prior to conducting a fusion I have found it expedient to scatter some bone ash powder on the furnace floor to absorb boil-over’s and leaking cupels.

In my opinion it’s a good practice to conduct a “blank” assay on/of the fluxes and any precious metal inquarts to be assured that no potential contamination has or is occurring. Thus the next 5 images are of the blanks.

Removing one of the test “blank” crucibles from electric oven.
I try not to allow the furnace door to remain open any longer than absolutely necessary to avoid thermal shock to the electric heating element.
Notice the white color above the fire brick, which are escaping gasses carrying metal oxides that create the sublimates on contacted surfaces.

This Blank Crucible charge, although very hot the molten Lead button can be seen through the molten fluxing agents.

Upon coming to cool the metal button is clearly visible through the slag at bottom of the Blank Crucible.

Some assayers, to save on costs will reuse clay crucibles, but I refuse to follow this behavior. Saving a few dollars and never being sure if the assay outcome was accurate due to the always potential contamination, is not, in my opinion worth the savings. Having said this, I do, however reuse graphite crucibles by pouring the molten metal and slag into cast steel mold as seen in the next 2 images. Nonetheless, even though the graphite crucibles are thoroughly cleaned by scraping out the debris there does remain some chance for contamination, but not nearly as much as with clay crucibles.

Something did not want to stay in the cast iron mold and shot out like a dart.

The “Blank” assay clay crucible is broken to determine the nature of the fusion. In this case it was a good melt and the metal button can be seen at bottom right with the slag residing on-top of the metal.

Notice how close the fusion came to boiling over the rim of crucible, which may have been due to a bit too much heat, too close to a heating element, improper fluxing or any number of possible circumstances, which this assay intended to illustrate.

The two blanks (one with silver and 1 without any silver) after the crucibles were broken and the lead buttons cleaned of any adhering slag were placed in bone ash cupels, which were loaded into preheated furnace that was 800F and the temperature setting then moved to near maximum temperature.

Not showing both Blank crucibles because they are identical in appearance, however there was a slight difference between the recovered Lead Buttons.

Left button weighs 29.5 grams and the right weighs 34.75 grams. Nothing else unusual about the appearance of the Lead Buttons. Why the difference in button weight is at best a guess, but fire assaying always seems to result in minor outcome differences. Nevertheless, proper fluxing is key to end results and the amount of the reducing agent (usually charcoal) is always the determinate factor in amount of Lead retrieved.

Both the Blank Button cupellations went well at 1725F. The pyrometer’s thermocouple in the oven is set down low to hopefully measure the temperature during cupellation. Whereas the actual temperature at the mid-way level in oven is likely 200 to 400 degrees F higher. Due to this fluctuation I usually try to watch the color of the heat rather than rely upon an analog temperature gauge.

Obviously, the silver inquart (in the right cupel) went through the fire just fine and the silver bead/prill measures = 0.052”, which was almost the same amount as was inquarted. So, no unexpected surprises.

Fortunately, there was no bead/prill from the other blank, which was expected and hoped for because it shows that no contamination occurred in the fluxes, which is very common with Lead Oxide (Litharge). In fact finding silver free litharge is hard to do as well as expensive to purchase.

The minor crack in right cupel is probably due to thermal shock of removing from oven.
Even though the Lead Buttons were clean of debris (slag) the cupels reflect some scoria, so I did not have the Lead Buttons as clean as I thought.

The silver bead in the “blank assay” was dissolved in heated 3 drops water and 1 drop of 70% Nitric acid and found to be of the same contamination prior to fusion. So, the subsequent silver inquarts originating from a 9999 silver Maple Leaf coin have same amount of minor contaminates, such as some base metals as well as tiny amounts of gold and pgms.

To reduce the amount of contaminates in any silver metal it is best to purchase 99999 or 999999 super clean silver metal or liquid, but be prepared to empty your pockets. For my assaying I use the 9999 1 ounce silver Maple Leaf bullion coins minted in Canada, which demands only another dollar or two over normal purchase price. It’s been my experience that all 999 bullion silver bullion rounds or flats are simply too filthy to incorporate in assaying.