How do you prevent pitting when doing a silver etch with ferric nitrate
The broader the areas that have to be etched, the weaker the solution needs to be. This slows the “bite.” The finer the lines, the faster the “bite,” so you increase the strength of the solution. Too strong a “bite” in large open areas will result in undercutting and pitting. Try a 15%-20% Baume’ solution and see if that solves or reduces your problem.
I can’t find any information on baume’ solution? Is there another name?
As I remember from college chemistry over 50 years ago, I believe it is a measure of the solutions specific gravity.
I googled “Baume’s Solution as it applies to etching silver”. Tons of information.
(Under the general heading of ‘Maybe TMI’) (No chemist uses baume unless they are 90 years old)
The maximum solubility of Ferric Nitrate Hexahydrate (FeNO3 - 9H2O) is 150 grams/100 grams of water which gives you ~ 36% FeNO3 at room temperature. Ferric Nitrate Hexahydrate (pale lavender crystals) is what you get from the supplier.
If you add 500 grams of Ferric Nitrate Hexahydrate crystals as received to ~1.2 liters of water (~39oz) you will get an 18% FeNO3 solution. Which is a pretty good place to start.
Note that if you heat the water somewhat, dissolution of the crystals will proceed faster as the dissolution of these crystals seem to be endothermic (opposite of exothermic or giving off heat).
You can also think about this…the rule of thumb is ‘by increasing the temperature by 10 degrees you double the reaction rate’. and visa versa…
So if you lower you solution temperature by 10 degrees (put it in the fridge) you can effectively lower the etch rate in half.
Additionally, when I etch, I use a magnetic stirrer and stir bar to help mitigate the effects of particulates that accumulate in the solution and to speed up the surface etch rate.
Finally, realize that as you etch you are consuming and converting the Ferric to Ferrous which not only doesn’t etch but also interferes with the ability of the Ferric Nitrate molecules gaining access to the metal…also slowing down the rate. Somewhat of a moving target.
My answer is to frequently watch the progress.
You can also use paint pens as a resist. The coloured ones work better than the black.
Sorry, I didn’t explain well enough. A Baume’ hydrometer is used to measure the strength of a solution by it’s density. I’m not quite 90 years old, but they were used extensively in photographic labs and darkrooms to measure the strength of the developer and fixer. They are STILL USED in labs for chemistry. Try picking one up through Amazon and using it rather than ‘by guess and by golly’ will improve your results considerably.
Hydrometers will work…both Specific gravity and baume…but remember that as you use the solution the baume will not reflect the activity of the solution…good to start though.
Thank you very much for your help, I appreciate it. I think I’m looking for a simpler solution, a ratio of ferric nitrate to water. My solution was too strong apparently. Thanks again you guys are worth your weight in gold
Thanks Don, I did find more information
Out of curiosity what is a magnetic stir and the bar.
I want to know what a magnetic stir and bar as well.
Not an endorsement of either of these brands but this is standard lab equipment. You don’t need to stir a lot, but provide enough movement to flush consumed etchant from the metal surface and allow fresh chemistry to reach the metal. Much faster and crisper than passive methods. Typically the industry sprays the etchant onto the metal.
I use the same stirrer for electroforming.
For me the biggest hurdle was getting the mask not to fail. Prepping the metal and getting a good bond.
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I built a Ferric Nitrate solution calculator in EXCEL but can’t upload it…
Oh my gosh you sound like me! I’m challenged when it comes to things like Excel. If you manage to figure out a way to upload the calculator I would love to see. Right now what I understand from all the help have gotten on the forum is that I need to use less ferric and not heat the water. Thank you so much for your help figure out this technique.
My prob here is Ganoskin will not allow uploading anything but images…probably for a good reason. So the only way to share this spreadsheet is to send it via email or share it on Google drive.
In general, and drawing on my decades as a chemist, heating a solution speeds up solution making, that is hotter water will dissolve most salts (in this case both Ferric Chloride and Ferric Nitrate) faster. There are some notable exceptions but not in this case.
But you really don’t need to know any chemistry to get this done. So the easiest way to do this is to give you a simple factor formula.
So a standard etching solution is to add by weight 33% more water than ferric nitrate.
For instance, if you had 100 grams of Ferric Nitrate crystals you would add 133 grams or mls of water. 200 grams of Ferric Nitrate crystals —266 ml of water. In this case water to crystals or visa versa - doesn’t matter.
So here is the tricky part…when you make this solution you will be slightly acidic with nitric acid and that is what you need. However, when you are etching you are getting two reactions.
The first reaction and most important is the reaction of ferric with silver to form soluble silver and also ferrous nitrate.
The second reaction is the reaction of the small amount of nitric acid in the solution with the silver metal.
This second reaction however is a problem as the acidity keeps the ferric nitrate from undergoing a conversion to Ferric Hydroxide, which is a solid and unreactive with silver. Thereby, shutting down your etching process. The answer is to add a small amount of acid to prevent this conversion from happening.
I use muriatic acid (hydrochloric acid) to adjust the solution but some purists would insist on using Nitric Acid which can be obtained through Rio( very expensive).
From a lay person handling standpoint each material has it’s issues…you can get a gallon of Muriatic from HD cheap (I use a 50% dilution in a 5 gallon plastic pail for cleaning steel and copper) but the concentrated solution is fuming and obnoxious. Masons use it to clean mortar off their work. Concentrated nitric acid is also fuming and much more reactive with living tissue.
Once either of these is heavily diluted they become significantly less obnoxious but the Safety Data Sheets are still your ultimate guide.
Again and I know TMI…apologies