Normally I add Nitric in very small quantities on a small batch of material where I know there's little base metal and little gold. If you know at outset that you have a very high base metal content such as Kovar and you know you have a good weight to dissolve i.e. 100g or higher, then you can actually premix your AR in a 5:1 ratio and get it hot. Start with something like 500ml of HCl and add Nitric to that 5:1 ratio. Then add your material a few small pieces at a time.
The reaction will go quickly! Extremely vigorous so do add a couple of pieces to "test the waters" so to speak. Your pieces will only take a few minutes to dissolve completely if we're talking about kovar lids. So measure the reaction and be careful and add your material in batches that suit.
I ran 4.5Kg of ceramics yesterday in 2 x 1.2 litre solutions as above and the time to completely process 4 procs in each was about 3-3.5 minutes comletely, so the whole batch didn't take long.
This isn't for newbies. It's for people who understand the AR process, who are kitted out for this, and who are processing reasonable to large quantities. The excellent "standard" processes for AR will suit most people better. Done correctly, there isn't a massive amount of excess Nitric at the end.
Remember your standard tests for running out of either HCl or Nitric, and remember your basic safety rules. Hot strong AR is very unforgiving.
