There are many excellent instructables on acid etching on here. So why add another one? Good question.
I'm covering acid etching on steel (a knife) and copper (for Xmas baubles). The technique is the same, the only difference is the dilution of the acid.
Tools and materials
Laser printer (inkjet will not work). Or you can use a photocopier. Toner is the important thing.
Laser printer PCB paper OR printer sticker sheets OR glossy magazine paper
Ferric Chloride acid (sold for etching copper circuit boards)
Acetone (or nail varnish remover)
Metal. Steel, copper or brass.
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Step 1: I Don't Want to Be Negative But...
Find suitable monochrome images. I picked some Christmas images for the copper disks, and some sewing images for the knife steel (I'm making my wife a tiny friction folder knife for her dressmaking).
Remember that the image will be reversed on the metal when the acid has done its job; both mirror image and in terms of being a negative image. So if you have text, make sure you invert the image. If the parts you want shiny are not the black parts, negativize the image. The Christmas image worked for me, but I had to negativize the scissors image. There are plenty of online sites that can turn your image into a negative.
Size the image to match the metal. I use MS Word as it resizes well. I copy the image several timesin different sizes. I then print on normal paper to find the image that fits the metal. Boom!
Step 2: Someday My Prints Will Come
We now have to print the image on the special paper. I have used cheap PCB transfer paper from eBay in this example. The important thing is that the paper is non-stick so that the toner will print, but sticks better to the metal than the paper. If you have a sheet of printer stickers you can take off the stickers and use the shiny side. You can even use a page from a shiny magazine.
As I have used PCB paper, I printed a whole load of images not to waste the paper. Make sure you use the shiny side.
Step 3: Any Old Iron
Make sure your metal is clean. Give it a wipe with acetone.
Set your iron to max and let it get up to full temp.
Cut your image out and place in the correct position on the metal. You can use masking tape to hold it in position.
Press the iron down for a few seconds until it sticks, then press hard and move the iron about for 2 or 3 mins until the toner sticks to the metal. You may start to see the toner through the paper.
Allow the metal to cool down then carefully pull the paper away. You should now have a crisp image. If not, wipe it away with acetone and start again.
Step 4: Mask
Now use a sharpie to to tidy up the image. You can also use nail varnish to cover all the metal that you don't want etched. Also cover the rest of the metal, the back and the sides. Only the bits you want etched should be visible.
Step 5: Acid House
Follow the usual safety steps for acid; gloves, specs and ventilation.
Prepare the acid. If you are etching copper or brass, use the acid at full strength. If you are etching steel, dilute the acid 1 part to 3 parts of distilled water (I used water for car batteries). Put the acid in a glass jar. Label it as you can use it numerous times.
Stick the metal onto a length of tape, face side up (duh).
Suspend it in the acid, using the tape rolled in on itself to stick to the glass.
The etching time will, of course, depend on the strength of the acid, the depth of etch that you want etc. My etch on both the copper and the steel took 30 mins. At 10 min intervals I took it out, dipped it in water *, checked and put it back.
When you are happy it is done, dip it in a glass jar of water with a tablespoon or 2 of baking soda. This will neutralise the acid and will fizz a bit. Dip it back in water to clean the baking soda off.
* IMPORTANT Do not use a stainless steel sink or I've heard the acid can stain it.
Step 6: Clean Up and Boom!
Dry the metal. Use the acetone to clean off the toner. You should now have a nice crisp etching.
Admire your handiwork.
In the first image are test pieces. Test steel for the knife images and copper disks for Xmas. I am happy with the steel depth of etch, but preferred the etched scissors, rather than the etched surround of scissors.. The copper images are okay, but I will etch them much deeper next time as the basis for enamelling (but that's a different story).
The second image is the tiny knife. Obviously the knife isn't finished and needs some tidying up. I also haven't shown the reverse of the knife as it contains sloppy images and love hearts that are between a man and his wife...