New key date and a tough hole filled

RomanTheRussian · Oct 16, 2017

Thanks guys. @TheFinn - the coin was gray and crusty until the acetone bath. The q-tip I used turned almost completely black. I would think improper storage or exposure to tobacco smoke resulted in the surface contamination that was removed by acetone. Quite the transformation.

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RomanTheRussian · Oct 16, 2017

@TheFinn - in terms of "plata agria" effect seen on many period 8 Reales, Swamperbob (Bob Gurney) posted the following info a while back on the coincommunity forums:

The fact is that silver was annealed (heat softened) at many points in the process from filet ingot to coin. That is simple fact that can be verified by reading period sources regarding minting procedures.

The fillets (the small bars cast to create a strip of silver just wider than one coin) were rolled to the desired thickness in a hand operated apparatus that used pressure between two rollers to thin the metal. It looked like a metal version of an old ringer washing machine. During the rolling process the ingots were actually annealed SEVERAL times. This was done so that the hand operated rolling machines would work easily and smoothly. Every pass through the rollers hardens the silver at the surface while the core is essentially unaffected. Too many passes of the rollers between annealing and surface cracking develops. That is the physics of metal working in silver. Silver will fatigue at the surface first while the core remains totally unaffected.

Fatigue cracking is a sure sign of a very low quality milling operation.

When silver is rolled by hand it is a VERY SLOW process. Speed either by narrowing the roller gap too quickly or by increasing the horizontal draw speed creates many problems. The cranking of the apparatus has to be smooth and steady - no pauses - no changes of pace. The mints of Spain did it right most of the time.
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After rolling the silver into strips, the Spanish mints laminated the sheet silver. This is perhaps the least discussed step in the minting process in the Spanish colonial mints. After the silver left the rolling area it was acid washed to remove dirt and debris and annealed again to produce as clean and soft a strip as possible.

Laminating is a process that was intended to REMOVE or reduce surface imperfections and irregularities.

Laminating consisted of drawing the annealed silver strip through a very accurate die to remove surface irregularities left by the rollers and to produce an absolutely uniform and smooth strip of metal.

You have to remember that every step which stresses the silver will harden and slightly FATIGUE the metal. These surface cracks are often a sign of simple metal fatigue.

After the strips are laminated and inspected and cleaned again they are blanked into round discs. The apparatus is a cookie cutter like device that also resembles a paper punch. This process hardens the edge at the point where the cut occurs.

The blanks are weighed, adjusted with files to the correct weight (face filing - not edge filing) and then they are washed and annealed again.

Then they are edged in the two flat bar edging mill. One at a time.

The final step is another bath, a weight check and a final annealing.

The finished planchet is now ready to be struck.
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RomanTheRussian · Oct 16, 2017

John Lorenzo had a different view on what caused the effect:

In my opinion this is called "Corrosion Induced Embrittlement" which is caused by improper annealing in the final stages of mechanically working a coin. See R.J.H. Wanhill in his research paper titled "The Embrittlement of Ancient Silver" and his work on the European Iron Age period Gundestrup Caulderon silver object. In my opinion this is strictly a physical deformation and not a contamination one - as we normally see Pb & Cu in 8R's - the copper is introduced to give strength to the alloy and the lead remaining from the cupellation process. Sometimes Pb and more often times high bismuth are considered contributers to embrittlement. Why then sometimes only a few pieces show this affect is simply due to to the fact that these were the pieces that were poorly annealed during the coin making process. The presence of lead may assist in this process. I just need to confirm this with microstructure analysis as performed by Wanhill in his study.
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RomanTheRussian · Oct 16, 2017

Opinions seem to diverge, but quite a few (myself included) believe the effect is due to improper alloying.

TheFinn · Oct 17, 2017

After working at a modern coining mint, I know that improper annealing of silver creates lots of problems. The mint I worked at was fanatical about the number of times the strips were heat annealed after each pass through the rolling mills and edge rimming. The results were no cracking, fully struck pieces and (with proper cleaning and polishing) NO MILK SPOTS.
Thanks for the two information bubbles on the cracking. After learning to pour molten silver and having some of it become brittle, I can appreciate what the smallest amount of impurities can do to the physical characteristics of silver.

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