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PCB's etching VERY slowly, muriatic acid and hydrogen peroxide

I recently started using the muriatic acid and hydrogen peroxide solution to etch PCBs.  All of the boards I do at home are simple, single sided, and range in size from 1-4 sq. inches.  The first board etched in a few minutes, the second 15 or better and now it is taking hours (after 5 or 6 small boards).  I have the fish tank pump set up to oxygenate the mixture.  The mixture is a nice green color, which from what I have read is correct.  Any ideas on why it is so slow now or suggestions as to what I need to do differently?

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These are all great suggestions.  The mixture has been getting pretty dark lately so it sounds like more hydrogen peroxide is called for.  I use a small ceramic bowl with about 8oz. of fluid, so I'll get out the coffee cup warmer I used when I was using ferric chloride and try that also.  Thanks.

Well, the MOST important thing you need to do is adding more hydrogen peroxide everytime you need to use it to keep the solution "fresh".

Also heating up  a bit like bdk6 said should help (although I actually never checked the thermodynamics myself) I never had to had to or bothered to do such a thing.

I prefer adding a bit of (kitchen) salt (also helps drive the reaction in the desired way) and it works for me.

Also I find that careful sideways agitation of the ecthing vat helps (it washes off the forming copper salts exposing more of the PCB's "fresh" copper to be attacked by the etching solution).