Forging Knives

Etching

Updated 4/24/08

For etching, we’ll need a liquid conductor, suitable resist and a power supply that can be regulated.

Liquid Conductor:

Saltwater will do fine as it’s one of the more conductive water-based solutions. The conductivity of drinkable groundwater is usually in the range of 30K to 60K ohms. A 7% solution of saltwater has a conductivity of something under 1K ohms which is adequate for our purposes. Increasing the salt solution further doesn’t dramatically increase conductivity.

Compared to acids and other etch methods, saltwater is safe, inexpensive and readily available.

Suitable Resist:

We need something nonconductive and waterproof which can withstand the mechanical action of etching and is easily removable. Traditionally, etching resist was some form of asphaltum. Lacquer spray paint/finish works fine--fast drying and is removed with acetone. Deft and Dupli-Color are two sprays which are suitable. Any lacquer-based spray product should work.

Lacquer seems to work best on ferrous based metals. It doesn’t adhere as well on brass and galvanized metals.

Finger nail polish works well as a resist.

Power Supply:

Save yourself the expense of a commercial DC power-source and just use dry cell batteries. Anything from 1.5V to 9V will work fine. The humble D cell will do everything you want done.

Increasing the saltwater solution and/or voltage won’t make much difference--varying either or both won’t hurt anything, but doesn’t help much either. 1.5V and 7% saltwater will do the trick.


To see how this works and to grasp the fundamentals, round up a 1.5V cell (AA, C or D--doesn’t matter), some leads, a glass container of some sort, a couple strips of ferrous-based metal--iron, steel, nails or some such.

Mix up some saltwater (1 tablespoon of salt in a cup of water, this is the same mixture we used while quenching), spray one of the strips with a couple coats of lacquer-based spray and let dry--a heat-gun speeds things along. Clean the metal with acetone before spraying to get a good bond.

Scratch what you want etched into the sprayed surface. Place the two strips (one sprayed, the other bare) into the saltwater bath and connect the leads. The positive pole of the battery goes to the strip you want etched--the sprayed one. The negative pole connects to the bare strip.

Metal attached to the positive (+) pole will be etched.
Metal attached to the negative (-) pole will produce bubbles.
Cathode is the negative pole; Anode the positive pole.

You’ll see bubbles emanating from the bare strip (cathode -). If they’re coming off the sprayed strip (anode +) then reverse the leads. You'll get a nice mark in minutes. Wait an hour for a very deep etch. Remove the strips from the bath, remove the sprayed material with acetone and inspect the etch. That’s it, that’s the whole deal! The rest is just refinement.

For this demonstration, use the same material in both strips. Using dissimilar metals can lead to strange plating/stripping events which may stop the flow of current. Bubbling at the cathode (bare strip) tells you the thing is working. After etching ferrous metals you’ll have some brownish pond-scum-looking-stuff floating in the bath.


Now for the regulation part:

We’re going to put a variable resistor into the etching circuit. These devices are called potentiometers (pot for short). Think of them as a faucet whereby you can control the flow of current. The volume control on your radio is a potentiometer. Introducing a pot allows adjustment for all the various variables--battery strength, salt solution, size of etching area and so on.

Below are diagrams for the front plate and wiring for an etching controller. It has a knob for the potentiometer--all good controllers have a knob--and two D cells are used in order to boost the voltage enough (3V) to light an LED. As long as we’re building this thing it might as well have a flashing light, although the LED is entirely optional and is added to the circuit mostly for amusement. LEDs have polarity and the long lead needs to go to the positive pole. Radio Shack part numbers are included in the wiring diagram.

The 5k pot should be connected so that resistance is high when the knob is in the low numbers. As the knob is turned clockwise (to higher numbers) resistance drops, more current flows and the etching becomes more active. High dial number, more bubbles, faster etching. I do most etching between 8 and 9.

In all etching, the degree of bubbling is the prime indicator with which to monitor the process. Saltwater etching as described above will produce a clean, crisp, deep etch without undercuts.

Download to desktop, print out, glue to 1/8" Masonite, drill and install parts listed below.