For those who haven't tried soldering, applying heat to your sterling silver causes it to darken with 'firescale'. The typical studio proceedure is to "pickle" the metal in a mild acid bath. There are also many commercial liquids out there, but I've never mixed them up myself -- I've always had a classroom environment that helpfully provided these (sometimes hazardous) chemicals. But it's tough to always have a class or a studio as a resource. So how could I solder and clean up those pieces safely?
The three methods I had heard about that were non-toxic were:
1) Lemon (only lemon, for the citric acid) Kool-Aid.
2) Vinegar and salt.
a ceramic cup
an electric cupwarmer
Pickle test was on: small 1/2" and 5/8" sterling discs that had sterling silver pieces soldered onto them with soft solder. Also fine silver components that had sterling silver components soldered to them.
I've now tried both methods and here are the results!
The idea here was to use a highly concentrated solution of lemon Kool-Aid and heat it like a pickling solution. I don't have a pickle pot (typically a small slow-cooker with a glass or other non-reactive heat container inside). I just use a ceramic mug and one of those desk cupwarmers to keep my liquids hot. I start by boiling some water and adding that hot water to the contents in the cup to get the heat going.
The citric acid in the Kool-Aid was to act as the pickle. While the solution smelled fresh and not chemical-y, the cleaning effect was slow and not very even across my firescaled pieces.
Buy some Lemon Kool-Aid on Amazon here! Kool-Aid Soft Drink Mix - Lemonade Unsweetened, Caffeine Free, 0.23 oz/envelope (Pack of 12)
Vinegar and Salt
The trick here is to use a saturated solution. I heated the vinegar to almost boiling, enough to put about 2" deep into the ceramic mug (enough to cover my silver pieces). To this hot vinegar, stir in salt until it no longer dissolves. This means the vinegar is completely saturated with salt. The vinegar fumes can be strong, so be careful!
When you add your items to this solution, you'll see a reaction immediately -- white foam and if your firescale is light, it cleaned immediately for me. Success!
I heard about this product through fab website Beaducation.com. It's a past and biodegradable, safe for the environment. While the product is meant for copper cookware, if you rinse your items well it should work well on brass and sterling silver! Purchase your own from Amazon EZ Brite Penny Brite Copper & Brass Cleaner / Polish 7oz by EZ Brite Brands
I've been using Pennybrite now for all my firescale cleaning. It's fast and easy and I love it. Wear gloves if you have sensitive skin.
Hope these options have given you some fun information on non-toxic pickle options for your metalworking needs!