Hi Stephen, argon is a good inert gas for topping up, especially given its density as you mentioned. I haven’t used it myself but I have spoken with some commercial wine makers about this gas and most have said that it is more costly than nitrogen so they often opt for that.

Argon and nitrogen are typically used by welders and thus the gasses can be purchased from welding gas suppliers. Nitrogen can be tricky because it is less dense than oxygen so you have to act fast when working with it.

Carbon dioxide works as well. You can drop a piece of food grade dry ice in the carboy and cap it with the airlock. As it dissolves the oxygen will be displaced. The drawback here is that you can introduce more carbon dioxide to your wine and end up needing to degas it later.

One of the easiest ways to give inert gasses a try is with Private Preserve. I’ve heard good things about this. They sell it at some wine shops for preserving open bottles or you can pick it up from wine making supply shops such as MIdwest (http://winemakersacademy.org/preserve (affiliate)).

I hope this helps Stephen! -Matt

Do you have any experience with this method and can you recommend one technique over the other? Wouldn’t carbon dioxide be as effective as argon, as it too is heavier than air? I wouldn’t know where to get argon from, but do keep Co2 on hand at most times.