From: REMOVE_THISdwilkins@means.net (Don Wilkins)
Newsgroups: rec.crafts.metalworking
Subject: Re: sulfuric acid .. silver soldering ..
Date: Mon, 20 Dec 1999 04:02:46 GMT

On Sun, 19 Dec 1999 13:15:39 -0800, Grant Erwin
<grant_erwinREMOVETHIS@halcyon.com> wrote:

>I'm about to do my first ever brazing attempt, using silver solder.
>Several respected authors suggest pickling in weak sulfuric acid for
>a few minutes, followed by a hot water wash while scrubbing with a
>brass-bristled brush. No one, however, has suggested a method for
>preparing such an acid solution for use. Boldly forging ahead on my
>own, I bought a jug of concentrated drain cleaner which says it's
>concentrated sulfuric acid. Took a sturdy glass jar with a tight-fitting
>plastic lid (jar about a quart) and filled it about 3/4 full with tap
>water. I put it on the bench, donned my face shield and acid-proof
>gloves, opened the jug of acid, and slowly poured it into the water.

You must have vigorous stirring to get away with this. I have done it
many times in the lab without a problem.

The other thing that usually happens the way you did it (equally
exciting) is the jar breaks and dumps the acid. This usually happens
with full compliance to all of Murphy's laws. If you are going to use
a glass container it must be Pyrex.

A much easier way is to weigh out the amount of water you need in the
form of small ice cubes. Pour the acid into the container containing
the ice. You end up with a fairly warm solution but it doesn't get
violent. I would recommend Pyrex here as well but you may get by with
ordinary glass.
>
>Let me tell you, it was scary what happened next! The entire
>solution gave a great burp, and plooshed about 1/4 cup of solution
>splattering on the bench and the floor!

What happens is that the acid flows to the bottom because of its high
density. You then have the acid-water interface to put out the heat
and the damned thing boils violently.

>I did have a box of baking
>soda handy, dumped that liberally, quickly made up a saturated
>solution of baking soda in water, and sprayed down everything until
>long after everything stopped fizzing. The jar was far too hot to
>handle with rubber gloves - I put the top on and, using tongs,
>gingerly set it out in the far corner of my lot on a bed of gravel
>to cool.

I am amazed that the jar didn't break and spill the acid.
>
>The lesson is, add the acid REALLY SLOWLY and be aware of what
>happens if you go too fast - it flash boils, and spatters acid!

I ran a magnetic stirrer full speed and with that you can pour pretty
fast. The key is don't let the acid and water form separate layers.

>I'm not aware of any serious damage to anything .. I feel very
>lucky. I plan to dilute this jarful about an addition 3:1
>before using it as a pickling solution.

Try the ice. It is the easiest way to dilute sulfuric.