How did the ancients plate lower value metal with silver? The Tetradr achm is one I just got and its plated.
This is my limited understanding only. Ancient Romans put a "silver wash" on certain coins, particularly the extremely-debased-silver coinage of the late 200s and early 300s AD. These are the coins usually referred to as "silvered'. If the coin did not see much circulation before being buried, this silver layer can be preserved intact. Exactly how the Romans performed this silvering trick is not entirely understood, and they may have used different techniques at different mints and different times. Some seem to have been made from blanks literally "washed" in molten silver prior to being struck. Other blanks have had a "pickling, a soak in acid to dissolve the copper off the surface and leave the silver behind. What is known is that silver coins still "looking silver" gave a "feel good" confidence, nobody was really fooled - the real-world purchasing power of the coins was not significantly greater due to such treatment.
That Tetradrachm isn't plated or silvered it is Billon. Billon is an alloy of a precious metal, in this case silver with base metals such as copper. By the time this coin was made the silver content had dropped below 50% and could well be as low as about 30%. That doesn't mean it is plated. These billon coins underwent a surface enrichment technique to make the surface of the coin appear more like a pure silver coin. We do not know the exact method used but there are many hypotheses out there. Techniques that are proposed include segregation during casting or annealing, deliberate thermal and/or chemical post-treatment such as pickling in acids or blanching. These techniques could also be used on much lower alloys such as that used in the much later "silvered" coins where the silver content was as low as about 5%.
Silver-plated coins are the fourrée denarii, this method was in use since a very early date. In the late 5th c. BC Athens, during the Peloponnesian War, they even minted silver-plated owl tetradrachms. In Rome fourrée denarii existed since the republican era. I don't think the late 3rd c. AD Roman antoniniani were silver plated (stricto sensu), they were silver washed, the bronze coin or blank was briefly put in molten silver. This operation left a very thin layer of silver on the coin surface. New mint-fresh coins looked like white pieces, but the thin silvering quickly vanished after the coin had circulated some time. I don't know if it lowered its purchasing power. Your Philip Jr tetradrachm from Antioch is not silvered : it is debased silver, but the silver content is enough for the coin to look white.
Here is one of mine that appears to have thick silver and it appears to be peeling off. It is a Probus with Quadrica.
Here is a Probus from the same period and mint. These were "silvered" and not plated. Here you can see where some of the "silver wash" has been lost showing the primarily bronze alloy beneath.
Process for making plated coins: There were multiple processes potentially used to make plated counterfeit and official coins throughout time. While there are several candidates for silver plating in ancient Rome, there is uncertainty about the process used for late Roman coins. Here are several candidates : "diffusion bonding" or "Sheffield plating" : silver copper sheets heated together reported in use for Roman Republican fourrée denarii, wrap copper flan in a silver foil and heat. This is considered too labor intensive to have been used at scale in late roman empire "dipping in silver chloride" - Blanks were maintained hot, dipped in molten silver chlorides, and quenched in a bath "mercury silvering", "amalgam plating", or "fire gilding" - a good way to get mercury poisoning - dissolve silver in mercury, then spread on the flan like peanut butter, then evaporate the mercury with heat, this technique is mentioned as a possibility for late roman coins. "depletion silvering", "pickling" or "blanching" - take a copper-silver alloy - use citric acid or vinegar to leach out the copper and enrich silver content of the outer surface, then strike to produce a silver surface plating which seems to be a leading hypothesis for later Roman coins. Ancient technology was a surprisingly sophisticated and included ploughs, irrigation, wheels, textiles, aqueducts, writing, ships, complex construction, and most relevant for this discussion: metallurgy of gold, silver, copper, and bronze. Bolos of Mendez in the second century BC describes methods for producing false silver: "About the making of “uncoined”: the quicksilver from arsenic, or sandarach, as you prefer, cook it as usual, deposit it on copper or coppered iron, and it will be whitened. Whitened magnesia does the same thing, and transmuted arsenic, and cooked cadmia (ZnO?], and unfired sandarach and whitened pyrite, and psimuthion [lead acetate] cooked with sulfur." For a collection of notes on ancient coins visit: http://www.sullacoins.com/
