September 17, 2012
I just built an anodizer as you described, but used an old Variac I got from eBay. I used a light bulb as a resistor across the capacitor, and wanted to replace it with a resistor. You specify a 100 ohm 200 watt resistor, but since you can use different light bulbs, I imagine I could use a resistor with different ohm rating as well. What would be the difference if I used a different ohm rating and what range would be acceptable?
If you have a variac (instead of an unstable and load-determined dimmer) then the only function of the resistor/bulb in parallel with the capacitor is to drain it, to reduce the voltage.
It is only needed when you turn the voltage down. My schematic shows the resistor with a series switch (use a momentary, normally open).
The resistance does not matter much, as it is dependent on the power produced and the time it takes are really the only things of interest.
Given up to 120v, you need to be able to handle the power of whatever the resistance is: (P=V2/R)
So a 120 ohm resistor needs to have a 120 watt rating at 120 volts.
But a 240 ohm resistor needs only 60 watts at 120v. Or 30 watts at 60 volts.
Also, as a momentary device you can use a lower power rating than if it were on continuously. The voltage (thus power) drops quickly, and the time to drain grows with the resistance (V2=V1 x e-t/RC) .
So my design suggests some middle-of-the-road options for resistors that are easy to find at electronics salvage places. And light bulbs are great because their resistance increases as the voltage does, so they drain faster than a fixed resistor. But they break.
May 9, 2012
I have a home made power supply that a friend built for me a long time ago that I use for anodizing titanium liners and bolsters on pocket knives I build, I don’t know much about electricity but I’m trying to learn, I just finished putting another unit together by looking at the one my friend made me. It has a variac, a full wave bridge rectifier, a light, a fuse link, a on/off switch, a momentary switch, an ammeter and a volt meter. Is it possible for me to add a switch somewhere to change the electricity coming out of my leads from DC to AC and Back. I’m wanting to acid etch my logo in the blades with the same unit if possible, DC etches the Stainless Steel deep and the AC blackens it. If possible, can you explain how to add a switch and what type of switch I need in layman’s terms, as a lot of the schematics look Greek to me. Thank you for your time.
The short answer is: This is a non-trivial change. One problem is that you need a different type of voltage and current meters for AC than for DC. So either you need high end meters that can sense the difference, or a duplicate set of meters and thus a separate set of output leads.
The simple solution would be to simply add another pair of leads from the variac output (before the rectifier) through another fuse and pair of meters to a separate pair of external jacks or leads that are clearly marked AC. And always make sure that only one set of leads actually leads anywhere or connects to anything. Use a separate 3pst switch (and signal light) to turn on the AC leads (presumably like the switch and light that connects the DC leads).
Basically, you can share the power cord and variac, but everything else would have to be be a separate circuit.
August 9, 2011
I have what I believe to be a piece of titanium that was found on a beach in Florida. I tested it using an acid test kit. The metal tested between 14 and 18k using a gold test kit. Should titanium test like gold, or has my acid possibly gone bad?
I’m pretty sure that the acid test bottles for gold use a variety of blends of acids, with Aqua Regia being the one that actually etches gold.
Titanium will be etched by these blends, as will platinum and a variety of other metals. I don’t know at what level each titanium alloy might test. I’d suspect that common Al6-V4 alloys would result in a lower karat rating than the commercially pure grades (#1-#4).
But the acid test will not tell you that it is titanium, as opposed to platinum or tantalum or niobium.
So I don’t recommend this as a reliable alternative to the suggestions I make in “How to tell if a piece of metal is really titanium.”
August 9, 2011
To niggle the semantics, it depends on what is common in ones world.
Any chemistry lab would have hydrofluoric acid, the fastest way to etch titanium. Its helper molecule sulfuric acid is available everywhere (battery acid or some drain cleaners). The combination of the two makes for a smoother etch, but you’ll have to ask a chemist, why?
I’ve found a blend of oxalic acid (HCO) and sodium bi-fluoride in a grocery store laundry section bottled as a rust remover. This etches the titanium, but can leave a carbon residue, that is easy to remove.
Supposedly, concentrated oxalic acid by itself could do the job. But I don’t see how from an entropy standpoint. Also, there is the risk of carbon monoxide fumes (oxalic acid is carbon-monoxide-acid).
ABF (ammonium-bi-fluoride) is common due to its high-volume use in the nuclear industry. I’ve used this by itself at high temperatures. It behaves like weak hydrofluoric acid; essentially buffered.
The key ingredient for etching titanium is loosely bonded fluoride ions. This means that anything that will eat titanium can kill you if it gets into your system. Some people are sensitive enough that a splash of HF on the skin can kill.
Some other suggestions and cautions are here: http://www.finishing.com/134/32.shtml
But my usual recommendation is to order Multi-Etch, a balanced blend of sodium-bi-fluoride and ammonium-sulfate, shipped dry and ready to mix: Visit http://multietch.com
June 19, 2011
I want to do a large piece of titanium (28″ x 3″ x .5″), around how many amps would I need to push through it, or how long would it take?
I have a power supply that is 12V @ ~19A, could I use this to color my titanium if I just leave it on for a long period of time?
If I color my titanium and dont like it, can I do it again and will I get around the same results? I’m scared that I will try to color it blue and get a terrible result and be stuck with it.
Okay, three questions, but the most critical one is in the title. Titanium colors are voltage controlled. A twelve volt power supply (or battery charger) would work for electroplating or aluminum anodizing, but not for titanium. More precisely, you can get the fingerprint-prone bronzes and deep purple at or under 12 volts. But not any of the other colors.
Because the final color is voltage limited, the current is less critical, in theory. In practice I find that to reach well saturated colors beyond about 50 volts you need a supply that can support an initial surge of at least 0.1 amps/sq.in. This can be done with lower rated supplies by charging a large capacitor in parallel with the electrodes. Your total piece is 184.5 sq.in, so 19 amps should be enough.
Burrs or sharp edges can have a negative effect on your final color.
If you don’t like your color, you can subsequently anodize to higher voltages, but not lower. The best color results appear on a clean and freshly etched surface. If you overshoot a color, or get a hazy or gray result, the only recourse is to grind, polish, or etch the color off and start over.
April 20, 2011
This question from Jack is a good one. I hadn’t really considered it before, and finding information on it online is either tricky or expensive. In short, I don’t know.
I have used electrolytes with a wide range of pH (acidity and alkalinity) but had not been looking for the differences. Some of my favorites are phosphoric acid (pH = 1.7), ammonium phosphate (4.2) , and tri-sodium phosphate (12). Quite different pH’s, but all slam that phosphate ion against the titanium anode and drop off an oxygen atom. Borates work fairly well, too. I’ve read that alkali sulfates can be used. But personal experience says, stay away from nitrates and chlorides.
According to some guidelines/requirements for anodizing titanium medical implants, a strong alkaline should be used. I suspect that this is to guarantee that nothing living can be in the solution.
According to one for-fee article from 1985 (Studies on anodizing of aluminium in alkaline electrolyte using alternating current) found “Electrolyte pH was found to affect the growth of anodic films considerably.” But I didn’t buy it to see how. This article is not quite to the point because it a) was about aluminum, b) they used alternating current, and c) it focused on only part of the pH spectrum.
If anyone has played with pH in anodizing titanium, please let me know if you notice any anodizing differences by pH.
April 17, 2011
I’m desperately trying to find a solution to my recently-developed metal allergy as I LOVE my earrings. Titanium seems like a great option to try, but I am wondering, what is the colored coating composed of? Is there anything in that I could be allergic to?
As I explain on pages such as A short article on the physics of Anodized Titanium Color, the color layer is pure titanium dioxide. Just oxygen bonded to the surface of the metal creating a material that has been used for artificial “Titania” diamonds.
Thus this layer actually is the hypoallergenic coating that makes titanium safe. The “silver” color has a thin layer of the oxide on it, whether I want it or not. Titanium spontaneously grabs oxygen from air or water to protect itself. When anodizing or heat coloring, the higher the voltage, the thicker the protective layer becomes. But there is no practical difference for hardness or sensitivity, as the thickest layer (green) is about 0.00000003 inches thick.
I use a simple phosphate detergent as my electrolyte, then soak the ear hooks in clear water. In the 30 years since I started doing this I have had only two customers who were too sensitive even for titanium wires. I suspect it was a non-chemical tactile or contact sensitivity; the rubbing or pressure itself was causing the reaction.
Some people claim that niobium is even better for sensitive ears than titanium. They are chemically similar, and involve the same method of coloring. It’s been discussed before here: Hypoallergenic: Titanium versus Niobium
I suggest trying both, as a spare pair of wires is cheaper than another shipping charge. Shop my Ear Hooks.