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.
But counterfeiters operating under more pressure often took shortcuts to speed up the process of planchet creation and those shortcuts left behind surface cracks and planchet irregularities caused in the rolling step.
That is a very simple fact that has been overlooked for a long time.
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.
Annealing in any hand powered metal working process is something that was learned in antiquity from silversmiths and metal workers in general. Annealed silver works better / easier. That should not be surprising to anyone.
But it is the die laminating process - which smooths the silver prior to blanking (actually removing the lines that were imparted by the rollers which were more crude than modern silver working techniques) that is most often forgotten today. The silver strips were pulled through a die that smooths out irregularities and leaves a perfectly uniform thickness. This step is rarely discussed and I am sure some forgers never even knew it existed. Lamination and the pressure developed at the die face would tend to close the cracks formed by rolling and since laminating was a one time process it very well may have smoothed out but not eliminated the surface cracks. These "closed" cracks would of course show up later in the life of the coin as circulation and handling cause toning to develop in the cracks.
In my opinion, the cracks observed today are more common on the silver restrikes made for the China trade, then on the genuine coins made before 1825 in the colonial mints.
Why you may ask?
The reason is simple and two fold - First and foremost, the advent of power equipment in the forgers operations meant faster rolling under HIGHER pressures. Manual rolling was of necessity low power and very slow but too fast a rolling operation will still produce chatter cracking. Even though the laminating step has been largely eliminated since modern machinery replaced the older hand methods - the higher speeds of rolling can cause problems and cracks. The second reason is the failure to perform the relatively simple step of die lamination of the bar.
I am not implying that all coins exhibiting this form of cracking are restrikes (or silver counterfeits) BUT A GREAT NUMBER ARE. The surface cracks are one clue I use in the identification of suspicious coins to test further.
If you notice coins made in haste during times of WAR exhibit far more in the way of cracks than do the peacetime coins. It is likely that during the war 1810-1822 no laminating machines were used at all except at Mexico City.
By the way, in MY opinion the 1804 is a silver restrike made for the China trade. The mouth shape is NOT good in addition to the surface cracks. It also uses a font for the number 4 that I have encountered before on confirmed Class 2 silver counterfeits.
I do not mean to imply that as a Class 2 silver counterfeit that the coin is worth less than a genuine coin of the same date. I believe that in the long run that such a large number of the Class 2 coins exist that the effect on overall pricing will be negligible, But in order to secure an ABSOLUTELY GENUINE coin made before 1808 it may be necessary to resort of high power scientific testing like XRF to be positive.