Creating the face wasn't the biggest technical challenge but it was the most frustrating to get something I was pleased with. I wanted the finished product to have an antique or aged appearance. Getting a good combination of aged-brass patina and readability was tricky and I ended up trying two different processes.
The first attempt was to engrave the brass plate with my cnc machine. I was able to make a workable face, but the finished product suffered in a couple ways. First, engraving text means profiling the letters. This creates a groove around the outside and around any interior 'holes' like in the top of a capital 'R'. Also, my homebuilt CNC router isn't accurate enough for this kind of work. Having a variation of only a couple thousandths of a inch over the size of the face means some letters would be engraved deeply and others would be faint. So, on to the second attempt.
The second (and final) face was made by electrolytic etching. I basically followed the process described in this article from the steampunk workshop.
More information available from Greenart.
One note: Some parts of this process are inherently dangerous. Please understand what you're doing before you begin and take care to dispose of hazardous chemicals properly.
The Magic Clock Face
1. I started by designing the face. The layout was drawn in ink scape. Mirror reversed and color reversed. Printed white on glossy inkjet paper with a laser printer. Uses LOTS of toner.
2. Clean the brass REALLY well and wipe down with alcohol.
3. Iron the printed design onto the brass plate. Iron temp is set as hot as possible. No steam. I put a blank sheet of paper between the photo paper and the iron.
4. Soak in hot water for a half-hour or so. This softens the paper. Begin peeling of the paper.
5. Keep working at this until all the paper is gone and only the toner remains. The exposed brass should be shiny.
6. Submerge it in a solution of copper sulfate.
7. Attach the power supply. The positive is connected to the face to be etched. The negative is connected to a similar sized piece of brass suspended near, but NOT TOUCHING. I used a battery charger. (Do not place your hands (or any part of your body) in the solution while the power supply is turned on)
8. Drink a beer...got to do something while you're waiting. The exact amount of time will vary depending on the strength of the copper sulphate solution and your power supply. I tested the process on a couple scraps and found 15 minutes to be about right for me.
9. Disconnect the charger (Important)
10. Remove the face and scrub off the toner with acetone.
11. You can see how much brass was etched away in this picture.
12. Spray paint black and let dry.
13. Sand off the paint from the high spots leaving the etched part black.
14. By making revision in the current, solution and sit time you can achieve different ratios of your antique look. For instance in my example the toner started to loosen up in places and allowed some place to be etched that shouldn't. Note the pitting in the corner and across the lower, left corner. I kinda liked the finished effect as it added to the antique look of it.