Why yes, you can freeze stock in Mason jars, which allows you to avoid saving your precious homemade potion in plastic. The only trick is being mindful of a couple easy yet essential tactics to ensure the glass doesn't crack. First, the stock needs to absolutely, incontrovertibly, and without a doubt be cooled to room temperature prior to pouring it into the jars. This prevents the shock of a hot liquid from cracking the glass jar. Second, when pouring the stock into the Mason jar, leave at least 2 to 3 inches of space at the top of the jar to allow space for the liquid to expand as it freezes. (It's preferable to use wide-mouth jars that are straight-sided as these tend to resist cracking more than jars with curved shoulders. If using jars the latter, fill each jar so that the stock comes beneath the shoulders of the jar, which is the part that gently curves inward just beneath the lids.) And when arranging your Mason jars in the freezer, be certain they aren't bumping against one another as the jostling that happens when you open the freezer door or rummage around for something could cause the glass to crack. (You may want to consider wrapping each jar in a towel or even a sock—a clean one, natch.)