Zap
This starts with empty glass, it's not done by printing or embedding. The 3D images are made of tiny fractures created by lasers: the beam shines into the glass without damaging it except at one focal point, where enough heat is created to crack the glass. These microfractures are about .1mm long, and they're pin-shaped rather than round, which you can see as you turn the glass pieces: the designs are sharper and dimmer in one axis, brighter and fuzzier in the other two. By convention we call the sharpest side the front, though sometimes there's reason to do it the other way.
To draw many points the beam is pulsed on and off, and repositioned by moving a CNC mirror. A typical design might use a few hundred thousand points; two million isn't unheard-of in a large block. The size of the marks depends on the frequency of the laser, e.g. a red laser gives marks that you can easily see individually. I'm currently using a green one which draws points that are barely visible to the naked eye.
The biggest piece I've worked on was a 28” square by 4” thick galaxy. The laser runs out of zap about 3½” into the glass, so that sets the limit on how deep a design can be.
The glass has to be optically clear, since any bubbles or inclusions would block or blur the laser marks. Images must be drawn in layers going from back to front of the glass, so old points don't block the laser from drawing new ones.
The front of the glass has to be flat so refraction won't blur and redirect the laser, and this makes it difficult to work with spheres and other curved shapes. There are tricks to get around this (e.g. immersing the piece in a glass vat of optically matched liquid) but it's work, so it's expensive. There are also viewing problems: in a 60mm sphere only about the center 20mm is useful, because magnification makes that area seem to fill the whole sphere. Anything drawn closer to the surface gets very distorted, and that doesn't leave much resolution to work with.
The origin of subsurface laser damage imaging, as it's known to its friends, is shrouded in mystery and lawsuits. Some hold that it was invented in the Soviet Union as a byblow of military laser research; others that a British brewery started it as a way to mark beer bottles indelibly. I'm agnostic and believe that everyone still trying to make trouble about this is a patent troll.
None of this is cutting edge: it's been mature as long as I've been using it, which is since 2002. My main supplier is Precision Crystal, they welcome inquiries. (If you're in Canada, I can also recommend Crystal Sensations.)
Design
Designing for this process is about getting the laser marks, which are individually almost invisible, to look like points, curves, translucent surfaces, solids. It's not trivial: a simple row of points doesn't look like a clean line and a plane of evenly spaced points doesn't give a nice surface. If you have a mesh model, zapping the vertices and hoping for the best won't work. You can't just pile up points till they look good either, since too many too close will overstress the glass and propagate a big crack which everyone will notice (this is called a "crash").
There's commercial software to solve these problems, but I've written my own which I think is better for what I do. I cracked a lot of glass getting everything working, and then there's the problem of persuading a laser facility to let you send raw point clouds. They don't really like it when you start cracking glass in their machines, but mistakes must be made.
My first idea with this was to draw sculptures in glass, but scientific imaging, and especially proteins, turned out to be a stronger application, and that led to my current selection. In some ways this picks up where 3D printing breaks down – it's great for data that's too detailed to print, too disconnected to be a single solid, too nuanced to show at constant opacity.
Have you got some interesting 3D data? Suggestions are welcome, and if I use it you get free glass and eternal credit.
Glass FAQ
I'd like to use one of your designs for an award or memento, can you add text?
Probably, I often add text and logos to designs at CrystalProteins. There's a setup charge, and also a single piece costs more than one of a large run, so the price of glass will be higher than stock. If this sounds reasonable, please write.
I want to buy a laser etching machine, can you advise?
I don't own or sell laser etching machines, so no. One manufacturer is Vitro.
Can you work from a photo or drawing?
I only work in 3D. Precision can etch photos for you.
Can you etch my car/house/child/dog?
I specialize in scientific imaging. Also, I also don't have a 3D scanner.
I saw one of these at a sidewalk stand, it cost $6.
Yes, there are many cheap imports, often with large laser marks. Mine are US-made with the highest frequency laser available, and also they are designed by me.
Can I get just light stands without glass?
A few is OK, but I'm not really in business to sell bases. I'd recommend Ebay or Amazon for a few, or Slee for wholesale.