Not to be too much of a wet blanket, but fine silver will tarnish also. Sulfur is the major culprit and I blacken fine silver all the time (and almost instantly) with liver of sulfur (sulfurated potash - a poorly defined mixture of potassium sulfide, potassium polysulfide, potassium thiosulfate, and probably potassium bisulfide available from almost any jewelry supply company.) (This reaction is the basis for niello work. Niello is a black metallic alloy of sulfur, copper, silver, and usually lead, used as an inlay on engraved metal for filling in designs. The Egyptians are credited with originating niello decoration, which spread throughout Europe during the Middle Ages.)
Copper and brass are also darkened with liver of sulfur, so sterling, while it may be a little more reactive, is not much different than fine silver. Sulfur, in whatever form, will combine with the surface of the silver and form a black silver sulfide layer. The only silver alloy I am aware of that is almost completely tarnish resistant is beryllium silver (~2% beryllium). There is also a relatively new alloy called "Argentium" silver - I have never used it, and I don't know what's in it, but it's supposed to be fairly oxidation (i.e. tarnish) resistant. Because silver tarnishes as it does, I do most of my wire inlay work with gold - it will never tarnish under any circumstances.
By the by, silver spot price is $13.93 today. If you think $19 an ounce is a high price for the sheet, try refining, pouring, rolling, and finishing a piece of sheet silver 0.064 thick by 4" by 6" for $5 extra an ounce. As much as I complain about postal rates, I still can't take a letter to New York (from California) by myself for 44 cents. I refine a lot of gold, silver, platinum, etc., for people tired of being ripped off by the large refining companies (they typically steal 30 to 50% of what you send them). But refining and processing these metals into useable new material is not a simple or inconsequential task. At $19 an ounce for new, high quality sheet silver, I think you got a great deal.