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Cleaning off etching sludge


I’m currently enjoying doing some etching. I first started off
etching sterling using nitric acid (at approx 25% strength). I had
the pieces with the design facing upwards and of course had to
contend with getting rid of the horrible “sludge” from the low parts
of the design. I didn’t much like the nitric acid for intricate
designs, as it gives too much of a chewed up look, being very
aggressive. So I’ve now switched to ferric nitrate, which gives a
much crisper etch. I decided to try and avoid the sludge, by using
foam, with the silver suspended underneath. However, there was still
just as much sludge hindering my etching, as there had been when I
had the silver the other way up!!! I’m a tad confused by this.

Anyway, I found it difficult to remove said sludge, and ended up
scrubbing it away using a wet, soft nylon toothbrush dipped in
bicarbonate of soda as an abrasive agent. It did the trick, but I
was wondering whether there’s a better, more effective way of
removing it, both during etching, so as to go on to achieve a deeper
etch, and without damaging my PNP blue resist, and after the desired
depth is achieved? What do other folks use?

Thanks in advance for any suggestions.



Being a printmaker (from my university degree, to tie in with
another thread!), we traditionally use feathers, and you brush the
piece as it etches. NOT being good at chemistry I can’t tell you the
scientific basis (I’m sure YOU can Helen, as you ARE good at
chemistry!) but while etching bubbles are produced, and they stick to
the surface of the metal, preventing further etch from reaching that
area (might have been part of your problem with the straight nitric -
or maybe you had it too strong?).

Anyway, if you will get some goose feathers and brush you piece
gently as it etches and bubbles develop I think this will solve your

I’m sure someone in the UK sells them - in the US I get mine from
Graphic Chemical and Ink, a major print supply house.

Beth Wicker
Three Cats and a Dog Design Studio



Anyway, I found it difficult to remove said sludge...What do other
folks use? 

There’s a good article on etching on Orchid.

Coral Shaffer describes using agitation or aeration, using an
aquarium aeration pump to assist.

I’ve also considered floating the metal upside down in the acid in a
tall glass container, then putting the glass container into the
ultrasonic (that already has liquid in it, obviously) to assist with
the agitation. Haven’t tried it yet.

Good luck,


Sludge will ALWAYS occur when different metals are used in the same
bath. Each metal requires a separate, new, bath.

To etch sterling silver, fine silver, bronze, brass, and nickel, The
Best Formula using Nitric Acid (69 to 70 % strength) is a mix of 2/3
part water to 1/3 part acid. For clean,sharp, etched lines, a
gentle, constant, agitation of the bath will dissolve the small gas
bubbles that form on the metal’s surface.

Etch time depends upon the temperature of water and the age of the
acid with etching time within a 10 to 20 minute period to attain the
desired etch depth. Although Ferric Nitrate renders an equally clean
etch, it does requires much more time to achieve the etch depth.

I have a complete " ETCHING " handout if desired.

Mary Ann Scherr


You need some method of agitating the etchant, stirring with a glass
rod or using a fine hair brush will keep the sludge away. You will
need to paint all the metalwork etc with a suitable resist [lacomit
is what I use] otherwise you will get contamination.

Nick Royall


Etching with ferric chloride or ferric nitrate is a redox reaction.
You are oxidizing the material you are removing and reducing the
ferric ion to iron. This is the sludge. It forms in the etched area
and the remaining solution is inactivated. This slows the etching
progress. Agitation provides the cure- this can be provided in many
ways. The feather technique,a sloshing tilt agitation of the etch
pan,pumping the etch solution around etc. large scale printing
plates and printed circuit boards are often spray etched with the
plates placed vertical. The Solution is re circulated pump sprayed
over the plate and drains into a sump. The Edinburgh etch improves
the process by chelating (tying up) the removed iron precipitant
permanently from solution. I prefer the electrochemical etch process
that can use many electrolytes. Just NEVER USE A CHLORIDE ( TABLE

We have been over this before this year. or Ask again if you can’t
find the posts in the archives.



Hi Helen:

My trick has always been to use a magnetic stirring unit to keep the
bath agitated. You can get them reasonably cheaply from university
salvage yards, or “used equipment” suppliers.



Hi Helen

I’ve done a fair bit of etching on silver and I always use ferric
nitrate solution with the design upwards so I can monitor what is
going on. I keep the solution quite shallow so it is well covered but
not in too deep. I very carefully brush away the sludge with a fine,
soft paint brush, two or three times during the etching process. I
wonder if it is possible to hang the pieces in the solution
virtically would the sludge simply fall away on it’s own? I do find
that the edges are not as sharp as I would like so if anyone else can
add to this thread and give advice on that problem as well I’d really
appreciate it.

Isn’t it fun?!

Collette (UK)


Vibration is the key…

Put your etching container on top of something that vibrates (a
friend puts hers on top of her clothes dryer and then runs the dryer
while her pieces are etching). The vibration is enough to make the
sludge drop to the bottom instead of clinging to your metal.

My crude but effective set-up is to put a glass Corning Ware dish on
top of the base of my rock tumbler and run it while etching. WARNING:
If you do something like this, put a piece of non-skid rubber in
between the dish & the tumbler to prevent the glass dish from
vibrating off the base.

Lauretta Bell

I found a very informative site with comprehensive info on etching: 

I have not tried the products they sell and I do not endorse them. If
anyone has used these products, please share your experience.

Richard Hart G.G.


My method which seems to work OK is to suspend the thing I’m etching
from a small block of expanded polystyrene foam so that it hangs free
in the etchant and I then have a small fish tank air pump bubbling
air through a length of plastic tubing into which I have drilled a
series of holes. This lies in the bottom of the etching tank which is
often just a cardboard box lined with a polythene bag. When I’m
finished etching, I leave the tank to stand for half an hour or so
for the sludge to settle at the bottom and then decant off the clean
etchant into a bottle and just throw the whole tank away. Easy, clean
and cheap!

Ian W. Wright
Sheffield UK


It seems that the consensus of opinion is to agitate the etching
solution, so thanks to all who have answered my question. I’ll have
to think about how best to accomplish that. I will hopefully come up
with something, because if it’s left, it’s really difficult to

Thanks everyone.



Thanks for the advice Ian. I originally thought suspending the
pieces would be enough, but it clearly isn’t, so I will have to rig
up something to agitate the solution.



A traditional way for small-quantity circuit boards is a rocking
tray. Photo processing trays are used, and placed on a dowel so they
can be rocked back and forth.

Al Balmer
Sun City, AZ


Back in the old days, my Dad used to process his own black and white
photos. These also needed to be agitated during the developing
process, and we used to use an old, very warped 78rpm record on a
very old HMV hand wound record player. Worked a treat, too. The very
edge of the developing tray rested on the outside edge of the
record, and as the record turned, the liquid in the tray was agitated
just right by the warp!

Jane Walker

These also needed to be agitated during the developing process, and
we used to use an old, very warped 78rpm record on a very old HMV
hand wound record player. 

That’s an interesting tale!

I’m tempted to go with the ultrasonic idea though, which a couple of
people have suggested to me. I’m hoping to give it a go tomorrow.

Thanks again to you Jane, and all others who have come up with some



I guess I need to get all my etching stuff out and give this a try
soon! Finally upgraded my printer to a laser so I can print my
designs onto PnP. Have several feathers (crow and raven) that I’ve
collected in my yard, the chemicals I need, containers, baking soda,

I’ve been collecting on etching silver with ferric
nitrate for quite a while now. This reply post is mainly to ask Mary
Ann Scheer (I have read and re-read everything I’ve been able to
find that she has written in books!!!) for a copy of her etching
handout. Please send me one. My email is @Susi_Wells1.

Thanks! And, wow, but I love ganoksin! It almost always “makes my

Au revoir!


I have always used a bubbler from a fish aquarium in my etching bath.
It cleans the sludge really well, the bubbles helping the metal to
fall to the bottom of the pan as it etches. I use heavy tape to
suspend the etching piece upside down during its etching bath. It is
a cheap and effective “helper” as far as time in the bath goes too,
depending on the strength of the solution. It does tend to work best
with the Sodium persulphate.

Good luck.