My sister Heather and her husband Luke recently opened a restaurant in Denmark (that's Denmark the small country town in the south west of Western Australia; not Denmark the small country in western Europe). They asked me to engrave the restaurant logo onto some ex-wine bottles that they could use to serve cold water.
The tool of choice for glass engraving is, of course, the Light Industrial Tool Of The Gods, the Dremel rotary tool. It's like an engraver on steroids. You can get a zillion different attachments for it, and use it for a zillion different things.
To engrave onto glass, you need a diamond bit. My local hardware store doesn't carry the official Dremel bit, but instead stocks the Aarlec equivalent. The only oddity with this is that it's a different size -- the basic Dremel kit comes with a range of 3mm bits, and a 3mm collet. (A collet is kind of like a drill chuck, but more compact, and not size adjustable.) The Aarlec bit is 2.3mm. To make it fit, you can either buy the 2.3mm collet, or do what I did, and put a bit of Mintie wrapper around the bit to pad it out.
The Dremel is variable speed, from about 10Krpm to 35Krpm. You need to use the highest speed you can; about 4 or 5 on the Dremel's 1-to-5 scale. This is because lower speeds force you to press the bit harder against the glass. This causes the bit to wear out faster; an expensive fact which I learned experimentally. But once you have the Dremel running at an appropriate speed, just touching the glass with the tip leaves a mark; it's like using a very fine felt-tip pen on paper. Except that felt-tip pens are lighter, thinner, quieter, don't vibrate, and don't require the use of safety goggles.
Speaking of which: safety goggles. Good Thing. Cheap safety goggles (which is all you need) cost about $5. What value do you place on your eyeball? Use safety goggles. I really mean it. Eventually something is going to get thrown off from the action end of the Dremel at warp speed (Dremel cutting wheels are particularly good for this), and you don't want it to intercept your eye.
Unless you're planning to draw free-hand on your bottle, you need to get some artwork placed inside that you can trace over. If you're engraving onto something where you can get easy access to the other side of the glass, like a drinking tumbler, this is easy -- print your artwork, cut it out, and sticky-tape it to the inside of the glass.
For a bottle, it's rather trickier. In the picture at the right, you can see my paper artwork inserted into the bottle. It's got a long paper handle at the top, leading out the top of the bottle. Behind it is a piece of card with a corner bent at a strategic point. The concept here is that you roll the paper artwork up small enough to fit through the neck, pop it in, slide in the cardboard prop in the folded position, push the prop down until the end of it is against the centre of your artwork, then pull it back up until it's snug against the other side of the bottle (just like in the picture). In practice, things tend to be less smooth than this. For a start, your rolled-up artwork doesn't automatically unroll once it gets inside the bottle. It tends to stay somewhat rolled, which means that even if you pin the centre of the artwork to the glass with the prop, the edges may still be sitting away. So you roll the artwork paper backwards, with the art on the inside. Then, once you get it in the bottle, provided you can get it to unroll at all, a single prop will be enough to hold it against the glass. In order to get it to unroll at all, you generally have to poke around with some sort of long stick-shaped thing, like a stick. Through the narrow neck of the bottle, this tends to be fiddly.
Once you've got the artwork unrolled, you have to get it in the right place. Modern glass bottles are made in a left half and a right half, which are then stuck together. This leaves a faint line going up and down each side of the bottle, along the join. If you're an obsessive perfectionist (like moi), you want to arrange your artwork so that it doesn't have one of these nasty seam lines going through it. You also want the artwork at about the right height, and you also want it level. If the paper is sitting too low, you can pull on the handle sticking out of the bottle neck. If it's too high, you can tap the bottle on the ground, and the paper will bounce down a bit with each tap. Unfortunately, as you pull and tap, the artwork may twist so it's not level. And the only way you have to control the levelness of the artwork is to pull, tap, and sort of twist the paper handle at the bottle neck. So getting the artwork at the right level and the right angle at the same time can be a bit of a challenge.
With the artwork in the right spot, you poke the folded prop down through the bottle neck, and when the end of the prop is sitting at the right spot, pull back so as to prop it against the other side of the bottle, with a gentle parabolic arch across the bottle, thus locking everything in place. Of course, sometimes the prop won't unfold, or, worse, it disturbs the position of the artwork, and you have to go back to square one. Getting the artwork and prop set up takes anywhere from 10 seconds to 15 minutes. And when it's more up the 15-mute end of the scale, you generally find that once you've got everything set up, it is necessary to put the bottle down, walk away, and spend some time extemporising on a theme of "Argh!".
Then it's the actual engraving. Make sure you've got plenty of light (I worked near-ish a window, and had a desk lamp right next to the work area), set the Dremel to maximum warp, and go for it.
The Capers logo fell nicely into three parts: the ellipse, the "capers", and the "restaurant". I usually did them in that order. Tracing through the glass is not quite as easy as it might at first seem, because the paper artwork is separated from you by the thickness of the glass (or more if the paper isn't quite snug). I found it necessary to work with one eye closed, and rotate the bottle as I went, so that I always had the piece of bottle I was working on lined up with the paper artwork.
As you work, the surface of the glass gets covered in a fine glass dust. Have a very slightly dampened sponge on hand to wipe this dust away. You want it to be damp so it will pick up the grindings, but still as dry as possible so that the glass will dry quickly after you wipe. This is because there is very high contrast between the ground and unground areas when the glass is dry, but when it's wet, the contrast is much lower, thus forcing you to wait for it to dry again before you can see what bits you've missed. Fortunately, the ground glass has a high surface area, so a trace of water will evaporate from it quite quickly.
I would have expected the glass grindings to be nasty and dangerous and sharp; but they weren't. I guess this has something to do with the way the diamond bit rips into the glass surface.
Once you've got the artwork all traced out onto the glass, you can open your other eye again, and go back and get the bits you missed, and do any colouring in you may have left. (Looking at the pictures, you'll see that the "capers" font has very broad vertical strokes; I usually did the outline and colouring-in for these separately.) Note that when you're colouring in, the direction in which you move the Dremel tip will be faintly visible in the final product. So for vertical lines, colour in with vertical strokes; etc.
There's no need to grind deep into the glass -- you only need to roughen the surface; you're not trying to create a 3-d effect.
One "gotcha" that did not become evident until the bottles were actually in use was that while on the dry glass the logo was clearly visible, "in the field" with cold water in them, condensation quickly formed on the outside of the bottle, dampening the ground glass and decreasing the contrast of the logo. Bah, humbug. For my next project, I plan to engrave some glass drinking tumblers. With no fiddling around to place the artwork, and the opportunity to use some "cartoonier" artwork that won't look bad if individual engraved lines are visible or go slightly awry, this should be comparitively easy.