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Laser engraving glass: a clean frosted mark, no chips

How to laser engrave glass with a CO2 or diode laser: which glass to choose, the damp paper trick, grey artwork to cut the heat, and what a diode needs.

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A CO2 laser marks glass with heat. The surface roughens where the beam lands, and the roughened patch looks frosted white. Too much heat in one place and the surface chips, or the glass cracks. So engraving glass well is mostly about heat: choose plain, even glass, use less power than you expect, spread the heat with a damp layer on the surface, and engrave the artwork as grey or as dots, not solid black. A CO2 laser marks glass directly. A blue diode laser needs a coating on the glass first.

Which laser

CO2. Epilog says engraving glass with a CO2 laser “produces a beautiful frosted effect”, and both Epilog and Trotec describe it as a routine job on flat glass, mirrors, bottles and drinking glasses. xTool’s guide gives the reason: a CO2 laser’s light “is readily absorbed by the glass”.

Blue diode. xTool’s guide says a diode laser’s beam is “not naturally absorbed directly by the glass”, so a coating is applied to make it work. xTool names two: laser marking spray, which is sold for marking metal, and tempera paint, sprayed on in thin even coats and allowed to dry. After engraving, the coating is rinsed off under running water.

In its guide to clear acrylic, xTool says a diode’s 450 nm light “cannot be absorbed by clear acrylic and other transparent materials” and passes through instead. A beam that the workpiece does not absorb is one more reason to keep the enclosure closed or goggles on. Laser engraving tips covers eye protection.

A cutting machine is no substitute here: Cricut says its engraving tools “are not intended for use with glass”.

Choose plain glass

Cheap glass engraves better than expensive glass. Trotec’s advice: “Cheaper, cast glasses are a good choice because they generally have lower lead content” and a more even structure. Hand-blown and crystal glasses “may have stresses in the material that are aggravated by the heat during laser engraving, which may cause the glass to fracture”. Epilog says the same of lead crystal, which it calls “more prone to cracking due to its composition”.

So practise on plain, cheap glass, and buy spares: Epilog’s advice is to have enough blanks to test on before the real one. Tempered glass, coloured glass and coated glass are not covered by any of the sources read for this guide, so test a spare piece or leave them alone.

Keep the heat down

Epilog calls too much heat “the number one cause of broken and fractured glasses”. The makers describe four ways to reduce it, and they can be combined.

1. A damp paper towel. Trotec’s method: lay a single layer of moist paper towel on the glass, with “no air bubbles or overlaps”, which would show in the engraving. Turn the air assist off so it does not dry the paper while the job runs. The result is “a clear, white engraving” with a smooth surface, and the paper wipes away afterwards. Epilog suggests damp newspaper in the same way, and warns that the moisture dries out, so re-wet it for a long job.

2. Dish soap. Epilog’s alternative is “a thin layer of ordinary dishwashing detergent”, which keeps the surface damp.

3. Tape. Masking tape or application tape over the area also works. Trotec notes the result is “rather grayish and not bright white”, and the glue has to be cleaned off after.

4. Grey artwork. With nothing on the glass at all, Trotec changes the artwork “from 100% black to about 70% gray”, or switches the engraving to ordered dithering at the same power and speed. Either way less heat goes into the glass, and “the surface is roughened less”.

And the plainest one: Epilog’s first instruction: “Use lower power settings on the laser to reduce the heat applied to the glass.”

Find the settings on a spare

There is no setting that suits every glass. Epilog: “Before running any glass engraving job, you must test your speed and power settings on the type of glass you’ll use.” Use an identical spare, or the base of the same piece.

  1. Start from your machine’s supplied glass setting, or from low power and high speed.
  2. Engrave a small filled square and a line of small text.
  3. Look at it dry, against something dark. An even, smooth frost is right. Chips and rough patches mean too much heat: lower the power, raise the speed, or add the damp paper.

If your software has a test grid, such as LightBurn’s Material Test (Laser Tools, then Material Test, in version 2.1), run it at a small square size on flat scrap glass of the same kind.

Round glass

Flat glass lies on the bed. A drinking glass or bottle needs a rotary attachment, which turns the piece under the beam. Epilog says rounded glass “will necessitate a rotary attachment” on a CO2 machine, and Trotec engraves curved glasses with one.

Two cautions from Trotec for glasses with handles: clamp the piece so that the handle never turns into the path of the laser head, and check that the design is shorter than the free part of the circumference.

None of the makers’ pages read for this guide describes engraving round glass without a rotary.

Photos on glass

A frosted mark is white, so the engraved parts are the light parts of the picture. LightBurn’s documentation says its Negative Image switch, which inverts the image, “is useful for engraving slate or glass, where burned areas become lighter”. Try a small piece both ways on a spare if you are unsure. Trotec’s recipe for photos on glass starts by setting the image to “70% black”: “This causes less heat to be applied to the glass, and the result will be finer.” It also recommends about 500 DPI and ordered dithering.

To prepare a photo in Plotcaster, choose the Dither style with Bayer 8×8, which is an ordered dither, and turn on Invert under Photo so that the dots land in the light parts. Export the PNG at the DPI you will engrave at. If you engrave the back of a flat pane to view from the front, mirror the photo in a photo editor first. Dithering for laser engraving explains ordered dithering and its alternatives.

Mirrors

A mirror is engraved from the back. Trotec describes “removing only the reflective layer on the back” with a fibre laser, which leaves the glass itself smooth. Epilog’s advice for its CO2 machines: “perform all the engraving on the reverse side of the glass”, flip the artwork so it reads the right way round from the front, and because the coating varies in thickness, “start with a lower output and increase it as necessary”.

After engraving

  • Wipe off the paper, soap or coating, and wash the glass. Epilog suggests a soft cloth for the residue.
  • To make the design bolder, Epilog suggests rubbing a coloured paint or wax fill into the engraved areas.
  • Engraving glass makes fine particles as well as fumes, and Epilog says both “must be safely removed from the workspace”. Keep the extraction running, and clean the dust out of the machine afterwards.

Questions

Why did my glass crack? Too much heat in one place, or glass with stress already in it. Lower the power, use damp paper or grey artwork, and switch to plain glass.

Why is the engraving rough and sparkly? The surface has chipped, not frosted. The cures are the same: less heat, and a damp layer on the surface.

Can a diode laser engrave glass without a coating? xTool’s guide says the beam is not absorbed by the glass, and its method uses a coating every time.

Is laser engraving the same as etching? Epilog names sandblasting through a template as the traditional technique, and xTool uses “etching” for etching cream applied through a stencil. For the laser mark itself, the laser makers use “etching” and “engraving” for the same frosted result.

Will it survive the dishwasher? None of the makers’ pages read for this guide makes a claim about it.

Sources