I have several glass carboys which I would like to permanently mark with gradations to measure liters and gallons. I've tried using permanent marker as well as heavy-duty tape, but inevitably both come off.
Any ideas for what I can do to permanently mark the glass?
For something north of electrical tape and south of glass-etching, I've used nail polish. Cheap and easy; even comes with it's own little brush. It won't come off in most circumstances, but it will if you take some acetone to it.
The problem with using anything that etches is that you're removing material from the unit. This can in turn affect the structural integrity in the long run. This is somewhat counter to why people prefer glass fermenters; they will resist scratching better than plastic. Etching is essentially controlled scratching.
A few years ago I took a bumper sticker and cut small strips and used them as gallon markers. I took my time and made sure there were no bubbles and got the corners flat. Its been like that for a few years without issue.
I'm impressed that permanent marker is being removed. I'm going to dip into art supply territory and suggest either screenprinting and/or pastel board adhesive.
Screenprinting ink is available at any craft store, but you'll need to make a stencil or freehand the numbers.
Pastel board adhesive is usually sold as an aerosol can -- it's used for coating pastel boards to keep the pastels in place (usually for chalk pastels). This should be usable on the outside of your carboy to protect any particular ink marking.
Have you tried Paint Markers yet? They're better suited to permanently marking glass, though for the best results, you usually have to bake it on. Enamels like nail polish can work well depending on the grade of it, and there are the Paint Stains that Martha Stewart puts out. DecoArt and Martha Stewart are the two brands I've used teh most, DecoArt being the better if I want translucent, and Martha Stewart's got some pretty nice opaque glass paints/stains.