Because of the structure of glass, the laser removes material by suddenly expanding the heated area — microcracks form, and if they propagate in one direction they can split the material. This is one of the most common problems for people engraving on glass.
If the power is too high (or the speed too slow), the local heat load on the glass rises above a critical level — the engraved area becomes "frosted," partly overburned, and for vector images there's a risk of cracking.
Glass is transparent — part of the laser passes through the material and reflects back at various angles. This scatters the energy and degrades precision.
Fill the bottle with water — the weight helps produce even rotation. Put in a paper towel if it can't be filled completely (the paper absorbs scattered light; water doesn't absorb 450 nm blue light).
Place a sheet of plain MDF under the glass. Do the framing on the MDF too — even light pressure from acrylic spray leaves a visible mark on the glass.
Glass needs to be masked with paint — the choice of paint fundamentally determines the effect you get.
If the paint has sufficient TiO2 content (matte white, off-white, ivory), the TiO2 method described for tile applies. The laser fuses the TiO2 onto the glaze — material addition, not removal.
Result has minimal relief effect. The image visually appears as "traditional black engraving."
Image processing: positive image, you burn the black. No need to switch to negative.
Black paint lets the laser light, on striking the paint, physically engrave the glass — it removes material.
Image processing: two steps are needed:
After engraving, the soot can be wiped off with a dry paper towel — this leaves the original dark surface, and then spraying the back white can further strengthen the contrast while also providing protection for the layer.
"Frosted," finer effect. This is the more favorable setting for photos with fine detail.
Bold, strong contrast — looks good on vector and vector-style images. Avoid on photos because of cracking risk.
Every glass type and thickness is different. Testing is especially important here — on a small piece, under identical conditions, find a safe speed/power combination before engraving a valuable piece.
Acrylic and tempera paint wash off with water — if the engraved glass will be near water, protection is needed:
Spray it with clear lacquer (preferably the same brand as the base paint's spray — this avoids solvent mismatch and re-dissolving). Both matte and glossy versions work fine.