For our purposes, finding a dewar can be tricky..... as has been mentioned, you want a "wide mouth" to accomodate the widest blade(s) you ever plan to do, herein lies the problem for using a dewar the way we do. Any dewar with a mouth larger then about 1 5/8" is considered a "temp" or "lab" dewar.....meaning that the insulation is such that its only meant to hold the liquid nitrogen for a very short period of time....generally less then a couple of hours. What that means for us as knifemakers is that we generally have to choose between a wide mouthed dewar that will accomdate the width of blade, and resign ourselves to refilling it every time we use it.....OR we settle for a dewar that has a smaller "mouth", that will hold/keep the liquid nitrogen for a longer period of time.
Personally, I have a dewar that has a 1 5/8" mouth, that holds 56lbs of liquid nitrogen (about 6.7 gallons). Its an expensive bugger to fill, so in order to try to "keep" the nitrogen as long as possible, I built a plywood box, with 2" polyfoam insulation on the inside. Its just large enough to place my dewar inside. Once the dewar is in the box, I fill all of the airspace with vermiculite, and place a piece of typical R30 fiberglass over the top of the dewar.....then close the hinged lid (the lid also has 2" polyfoam on the inside).
I'm limited to treating blades less then the opening size, however, once filled and in the box, I can maintain enough nitrogen in the dewar to treat blades for approx. 50-60 days. Prior to building the insulated "box", the dewar would be dry within 12-14 days, no matter the external temps.
When I treat blades, I hang them a length of stainless wire, slowly lower them into the liquid (lots of "boil off happening), and as soon as possible, put the plug on the dewar in place, and get the fiberglass insulation back on top, and close the lid. One thing I learned early on is that unless you want to get a lot of mico cracking on blades, you need to ensure the once removed, the blade are forced to warm up SLOWLY....... the first few blades I did... I pulled blades out of the dewar, and laid them on a steel bench.....bad move.....within a few seconds they started popping and tinging......and by the time I realized what was happening, it was too late...they had already cracked is a number of places along the edges. Since learning that painful lesson, I now take the blades out, and lay them directly on a large 1" thick piece of kawool.....and either fold it over the blade(s), or place a second piece of kawool on top. It takes about 1-2 hours for them to "warm up" to where I can handle them without welding gloves....but it solved any cracking issues.
