17 replies to this topic

[AvishaiW]

Safety first: Before starting handling corrosive acids, you must take all the precautions and read the safety rules about handling acid. Do it in a ventilated area, keep away from heat and open fire, when diluting acid always pour acid into water, never pour water into acid. Wear eye protection, avoid contact with the skin etc. The list is long but very important.

Hi to all,
I am going to demonstrate here how I do deep groove etchings on architectural items such as door handles and Judaica items. The basics of etching is very simple. The metal is etched only in the exposed areas that contact the acid. The masked area is painted with an acid resistant material that insulates the metal from the acid. Here is a pic of an etched item


The resist: The resist that I use is Asphalt Lacquer which is made of tar dissolved in mineral spirit (Turpentine). The resist can be applied by brushing, spraying over a mask or by dispensing. Here is the dispenser that I made.


The dispenser is assembled from a low air pressure regulator, a miniature needle valve, a pneumatic foot switch, some tubing and a disposable syringe attached to an adaptor.
Filling the syringe with the resist.  


I cut the syringe plunger and attach it to the adaptor.


Painting the resist: When I started doing etched handles I did it with a small brush- it took hours. By using the dispensing method the process is much faster, yet each part is individually painted which keeps it an artistic item. The line thickness can be controlled by changing the needle size, the needle tip and by changing the pressure and the air flow in the dispenser.
The part is held in a bearing housing


The pattern is painted using the foot switch to release the resist. The painting is exactly like using a fountain pen.


After painting and drying:


The etching process:
Again, you should take all safety means. I use a solution of 30% Nitric acid (HNO3) diluted in water- remember allways to pour acid into water and never pour water into acid!!!. Do it outside in a ventilated area. The process releases Hydrogen gas which when concentrated is explosive. In addition, a mist of the acid might be distributed. The first time that I etched, I did it indoors and in few hours all the steel parts in a radius of 6 feet got a nice rust layer. The etching process takes about 15- 20 min. depending on the acid strength and how deep you want the etch. During etching bubbles are caught between the metal and the acid, and act like insulators. A fine brush or a stream of air in the container agitates the acid and causes the bubbles to float. I also take the parts out of the etching solution every 2 min. to get rid of the bubbles. The photo shows the parts during etching (in a coffee jar)


and after 20 min.


After cleaning the resist with paint thinner. Note that the surface is lower than the masked areas and the borderline between the etched surface and the masked line is deeper.


After polishing


After patination and tumbling


Some finished items  


Please feel free to ask any question about the process or share your own experience. I am always happy to learn something new.

[ShawnM]

Avishai -

Excellent demo! I inserted a few new lines just to make the reading easier.

Thanks for sharing your process. The dispenser is very slick and end result is beautiful

[Alfredo Alamo]

AvishaiW.

Thank you for sharing your experience with us. The process is freaking cool[Punk]. I only have a question why is the borderline deeper than the edged and masked areas?.

[Tony Mertens]

Thanks for the demo.  I've heard other people explain the process here but the pictures help it make sense.  I have the same question as Alfredo.

[PTsideshow]

' /> great demo the only thing I will add is that here at some jewelery suppliers the resist is called resist or asphaltum.

And to remove the bubbles or agitate them you can use a feather to gently brush away the bubbles that form and interrupt the etching process.

First rate demo!

[AvishaiW]

Quote

I only have a question why is the borderline deeper than the edged and masked areas?.

In the borderline area, the acid solution meets the horizontal plane and the "vertical" wall that was created during the etching. therefore the total contact area between the solution and the metal is larger which gives a larger area for the acid to bite. I am not sure that it is the accurate explanation- that's my theory. I have noticed that when the solution is weaker, it does not happen.

[Tony Mertens]

That sounds logical, let's run with it.

[AvishaiW]

Quote

Excellent demo! I inserted a few new lines just to make the reading easier.

Shawn,
Thanks for the editing. It looks better now[Beer]

[fidget]

Hi

Answer to "a question why is the borderline deeper than the edged and masked areas?. "

Its caused by " cavitation "
From Wikipedia "Cavitation is the formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapor pressure. Cavitation is usually divided into two classes of behavior: inertial (or transient) cavitation, and noninertial cavitation. Inertial cavitation is the process where a void or bubble in a liquid rapidly collapses, producing a shock wave. Such cavitation often occurs in control valves, pumps, propellers, impellers, and in the vascular tissues of plants. Noninertial cavitation is the process in which a bubble in a fluid is forced to oscillate in size or shape due to some form of energy input, such as an acoustic field. Such cavitation is often employed in ultrasonic cleaning baths and can also be observed in pumps, propellers, etc.

Since the shock waves formed by cavitation are strong enough to significantly damage moving parts, cavitation is usually an undesirable phenomenon. It is specifically avoided in the design of machines such as turbines or propellers, and eliminating cavitation is a major field in the study of fluid dynamics."

[PTsideshow]

Believe it or not it has a name it is called the etch factor!

Etch Factor is the etch depth divided by the amount of sideways etch at an edge where the resist meets the metal.

An etch factor of 2 is means the etch is depth is twice as large as the under cut.

Agitation of the mordant(etchant) solution is a means of increasing the speed of the etching and it will control or reduce severe undercutting(lateral or sideways etch)

Using a feather to brush away the reaction and residual products, such as bubbles and sludge. Feathers are not affected by dilute acids, since when they are colored they use an acid solution.

The agitation of the gas bubbles are continually presenting new surfaces to the etchant, give a smoother surface finish. If you only agitate the etchant only occasionally you will get a mire textured surface. the gas bubbles and sludge tend to block the even action of the etchant.

Jewelry, Concepts and Technology by Oppi Untracht Has a great section on surface ornament with out heat. The sub heading The acid etching of metal, covers the standard type that is described in this thread, covers mixing up resists formula's for assorted treatments. Covers all the materials that are used in the process. Covers the acids used and the acids used for various metals.

Then he goes on to cover Electrochemical anodic etching.
Stencil etching metal sheet using light-sensitized gelatin.
Photomechanical one or two surface etch piercing.
Making photofabricated transparency as a stencil.
Halftone Photoetrching on metal.

I can not endorse this book enough, It covers the whole field of metalsmithing as does our forum. The title is a misnomer it is the wiki of metalsmithing and almost all older metal workers have at some time been captivated by it.

It isn't cheap even used, but it is worth every penny it will cost. With the knowledge it will impart.

His first book Metal Techniques for Craftsmen, by Oppi Untreacht

[Alfredo Alamo]

Thank you so much I want to try that!!!

[crquack]

Fascinating!

1) I think I identify several kinds of metal including steel and brass. Correct?

2) I have never seen the edge effect you have demonstrated, but then I use Edinburgh Etch and do not get anywhere near the depth you are getting, especially in steel. I wonder if the explanations offered are the entire story.

3) I like the dispenser! My etching tends to be "negative" (I scratch the pattern into a plate covered with resist - I found the Duplicolor primer works best for me). For your sort of positive pattern etching I found that enamel hobby paints work fine but the most convenient resist is this pen:

http://www.studioart...ePaintPens.html

On flat pieces I try to use Press-n-Peel whenever I can. I use the above pen for retouching if necessary.

When, like yourself, I tried the tar-in-varsol method I had some difficulty in getting a consistent density of the resist and there were areas of resist washout during etching (I notice some on your pieces but perhaps I am wrong). I have never tried the commercial asphaltum resist.

The Porcelaine pen does not come off at all.

4) Have you considered electrolytic etching? I guess if you get good results with Nitric Acid and do not mind working with it there is no reason to change.

I have had some encouraging early results with electrolytic etching of both brass (in acidified copper sulphate) and steel (in saturated salt solution) but got side-tracked before I could work out the kinks.

Of course I am motivated by acid-related risk-aversion :-)

[AvishaiW]

Thank you all for your inputs.
The cavitation sounds logical. I am not familiar with the term "etch factor". Will have to look for Oppi's Bible. I have only his first book.
crquack, I can not find those ceramic markers in Israel, but I tried few other types of markers and failed. You are right, sometimes the Asphaltum resist peels out of the surface and floats in the etching solution. I am doing also electrolytic etching or moreaccurate electrolytic polishing on small stainless steel tubing - I use it mainly for deburring after grinding injection needles (one of my products). but the etching in this case is very fine. What is Edinburgh etch?

[crquack]

Ah, I understand! I suspect importing things into Israel is probably even harder than importing them to Canada. FWIW the pens are made in France I believe.

I too have tried a bunch of other markers; notably Sharpies are quite useless.

I am more and more intrigued by the edge grooves! I note, for instance, that they are absent on the steel ball handle!

I had a look through a ton of industry literature. Although the etch factor is well recognized as a phenomenon associated with isotropic etching nowhere could I find a description of similar grooves. Cavitation has never been mentioned - if anything, bubbles of hydrogen tend to *obstruct* the etchant both by forming "bridges" and "snowballs".

I cannot relate the grooves to an undercut (although undercut maybe and almost certainly is present in your case) - undercut generally does not produce intensified action of etchant adjacent to the resist at right angles to the surface. Then there is the appearance of the grooves: They are not even, they have a sort of scalloped structure.

I cannot pretend I know the answers but like a dog with a bone I continue to chew on it :-)

Some links of interest:

Edinburgh Etch
http://www.ganoksin....inburg-etch.pdf

Etching in general (sorry, you probably know most of this)
http://www.ensc.sfu....495/e495l5j.pdf

http://www.pcbfab.co...ing-outer-layer

http://www.chemcut.n... Etching(Metric).pdf

- in that last one they discuss the "undercut ratio" on page 13 which is basically the etch factor

[bigfootnampa]

Interesting stuff and BEAUTIFUL work Avishai!  Thanks for sharing!  It may be a while before I try anything like this as there are already too many irons in the fire... but I like it!

[PTsideshow]

A word of warning about the ferric chloride solution, It will etch through any metal in the drains pipes/sewer system. Unless it is neutralized, as many printed circuit board maker has found out after the fact of. Not to mention that it is illegal in the states and most other countries  
This is a safer mordant that has come out of the printmaking arts and industry. the below are from the following link

Another word of caution (e.g., no etching of aluminum with ferric chloride)as a reaction between aluminum and iron could lead to explosive results.

There are many different mordants or solutions that can be used some work only with certain metals.

As with patinas and the chemical used when doing patinas, Use the proper personnel equipment and studio shop set ups. no open flames, and the proper ventilation.

Metal Salt Etching: Basic Recipes

Saline Sulphate Etch
To etch zinc, aluminium, mild steel

Mix 100g copper sulphate crystals
with 100g cooking salt
dissolve in 1 litre of warm water

Edinburgh Etch
To etch copper or brass

Dissolve 250ml of citric acid crystals
In 1litre of warm water
Mix with
4 litres of saturated ferric chloride solution
(strength about 40%, or 42-48 BE)

(multiply or reduce amounts while retaining correct ratios)

The father of the Edinburg Etch.

more on the safer etches

And as with patinas, etching is controllable by the S-T-T

Strength: of the solution of the mordant amount of acid to water in it.

Time: weaker or stronger solutions will have an affect on the etching effect of the piece.

Temperature: Hot or warm are more active than a cold solutions, but with it brings a whole new group of things to deal with the mordant solution.

Disposing of Ferric Chloride:
The solution must not be put down the drain because of residual copper ions left in it. To make it safe for disposal, you can add sodium carbonate (washing soda) or sodium hydroxide to it to neutralize it, until the pH value goes up to between 7.0 and 8.0, testing it with indicator paper. Copper will be deposited as a sludge. Allow the sludge to settle, pour off the liquid, further dilute it with water and then it can be poured down the drain. Collect the sludge in plastic bags and dispose of it as required by your local waste authority.

I also have to agree that the use of the term "Cavitation" isn't correct to describe the effect of the bubbles. As it is dependent on flowing liquid at a higher flow rate that causes the vapor pressure effect. A fluid is forced to oscillate in size or shape due to some form of energy input, high rate flow of fluids in a pump or piping valves etc.

Even in chemical engineering applications, where cavitation is often used to homogenize, or mix and break down, suspended particles in a colloidal liquid compound such as paint mixtures or milk. Many industrial mixing machines are "based upon this design principle". It is usually achieved through impeller design or by forcing the mixture through an annular opening that has a narrow entrance orifice with a much larger exit orifice. In the latter case, the drastic decrease in pressure as the liquid accelerates into a larger volume induces cavitation.


The standard disclaimer: A whole lot more guests and non participating members, that may not have the knowledge about the hazards of some of the processes, and materials read these posts.

[AvishaiW]

Thanks again for your inputs. It will take some time to read the sites that you mentioned.
Crquack, the ball handle is also made of brass, but lighter colour. I am unable to check it since it was sold, but as far as I remember, it had the same grooves. It was etched togather with the large cone handle at the same parameters. The scalloped structure of the groove is due to the bubbles caught during the process. Regarding the Sharpie pens, I had the same experience that you had - it peeled off after a minute.

Glen,
I never used Ferric cloride. I use Citric acid for electropolishing stainless steel. Regarding disposal of used acid, It is illegal to dispose diluted acids down the drain because it carries metal ions which will kill the bacterias used to process the drain water . I neutralize the acid with Caustic soda and keep the solution in an open top plastic container which is locked in a cage outdoors. When neutralized, the solution evaporates quicker, leavins some salts at the bottom of the container. When there will be a certain quantity of salts, I will look where to dispose it. (Maybe in the containers designated for used batteries- I'll to check it with the environmental services)

[Randy McDaniel]

Amazing and beautiful! Thanks for sharing!!!