All of the restaurant stainless I've bought has been non-magnetic.
I've run many, many 2# batches of large sweated-off contact points (not any containing tungsten - I sorted all those out first and ran them together later) in 5 gallon buckets and never found it necessary to heat the nitric solution. I usually only did 2# at a time to keep the bucket from getting too hot and to allow plenty of room for foaming. It also kept the solution depth below 2" which, in my experience, is about the maximum acid depth for efficient dissolving. The reaction produces so much heat that you can hardly lay your hand on the side of the bucket yet, not so much as to melt the bucket, although it did soften a bit. I put the bucket in a tray to catch any drips, put it under the fume hood, added 2# of points, calculated the nitric and distilled water I thought I needed (for a 50/50 solution), covered the points with all the water, and added about 1/4 of the nitric. The reaction started quickly. When it died down, I gave it a stir and added another 1/4 of the nitric. I repeated this until all the nitric was in, which took a total of about 30-45 minutes. I then loosely covered the bucket, put it at the back of the fume hood, and let it work overnight.
Since sweated off contact points usually ran about 82% silver, with most of the remainder copper and cadmium, it took about 1.75ml of nitric and 1.75ml of distilled water to dissolve 1 gram.
The next morning, I poured the solution into another bucket. If there was any undissolved silver (usually very little, if any), I dissolved it in a beaker with just enough fresh 50/50 nitric, using a little heat, if necessary, so it would go faster. I then combined the solutions, added an equal amount of distilled water, stirred it, and filtered it. I put the filtered solution into a clean bucket and added at least 6-8 lengths of bus bar (2", if I could get it) separated from each other as much as possible. I stirred it occasionally during the day with one of the bus bars and then let it work overnight. I tested it for completeness in the morning with a drop of HCl. It was usually finished. If not, I let it work more. When it tested finished, I scraped the bus bars down with a plastic scraper, rinsed them with a squirt bottle, and removed the bars. I let it settle, siphoned or carefully poured off the tops, filtered the tops, and then filtered the bottom residue in the same filter(s). After thorough rinsing in the filter, I dried it, melted it into cell bars, and ran it through a silver cell.
