Lasering a mirror, what are these markings and how do I prevent it?

[Network23]

Halp!

[Network23]

Still can't figure it out or find anything with the googles but did find it helps if I do a horizontal scan and a diagonal.

[Zax]

It looks like different thickness of material or at least different density causing the pattern. Once you remove it fully it should be OK, which I think is what you achieved in the latest image.

[ggallant571]

I do about 5 mirrors per week and have no idea what your first 2 pictures represent. If you post the design file I will examine.

Some notes:

   1. There are at least 2 layers of coatings on the back side of typical mirror.
   2. It is possible to burn off just the reflective layer and not touch the glass.
   3. It is possible to fracture the glass.

You can get a 3D look by just burning the paint and reflective layers, leaving the glass untouched, and applying more heat elsewhere to get a frosted appearance. It takes testing and varies with each vendors product.

[Network23]

Then this may make sense.

First mirror I laid it on my burned MDF board and the burns on the board came through.

Put it on top of a white sheet of paper, and got those lines.

Placed it on a piece of ply and it looks MUCH better.

Having it running at 2500 diagonally and its looking smooth.

[ggallant571]

I don't understand the square wood looking pattern. Are you burning thru the mirror or from the back side? I usually put a piece of white paper on the waste board to protect the surface from scratches. Suspect it helps keep the char from the waste board from getting on the surface.

What power laser do you have?

[Network23]

2.5w.

But putting it on a piece of ply seemed to fix the problem.  Yes, I am burning the back.  White paper gave me those lines.

[ggallant571]

Do you have random low intensity junk in the design file that only burns the mirror when etching slowly?

[Network23]

Problems fixed. Seemed to be the material it was laying on.

[ggallant571]

How!!!!

[Network23]

How!!!!

 ¯\_(ツ)_/¯
All I can figure is it was caused by what I was laying it on.  Still had some markings, but ran 2 passes diag and horiz, came out perfect. Speed 2000

[Zax]

That looks fantastic, well done.

[laser_cutter]

Then this may make sense.

First mirror I laid it on my burned MDF board and the burns on the board came through.

Put it on top of a white sheet of paper, and got those lines.

Placed it on a piece of ply and it looks MUCH better.

Having it running at 2500 diagonally and its looking smooth.

I have complete about 10 laser jobs on mirrors.  I keep the mirror about 3mm off the surface of the table. I think what is happening is what I will call a double burn - where the laser is burning two surface. Try place scrap pieces of craft plywood or coins to keep the mirror off the table.