I’ve not measured the depth yet but I didn’t go as deep as I would have liked. I was giving it a wash and a gentle rub and the paint started coming off so hopefully it’s deep enough. I plan on filling it with coloured epoxy then varnishing it with a special varnish for brass called Incralac.
Well, I have A result that is successful, though its only after a bit of elbow grease.
I dont have the mirror remover yet, because it has to be ordered in from abroad, so I only had methylated spirits, rubbing alcohol and steel wool to clean the back.
I put a layer of masking film on top of the backside, and set the layer to 500mm speed, 30% [of a 100W rated tube] power and with 4 passes.
After washing the residue off and cleaning most of the mess I was still left with a lot of reflective material, so a good 20 minutes of scrubbing it, with the masking film still on the back to prevent it from spreading and damaging the back, I was left with a good clear circle shape.
Not the magic solution but, if it works, it works.
You burned away the masking film then?
If so, good idea as a way to protect the areas from the scrubbing you do. A chemical treatment should save your supply of elbow grease.
I did yes. though I think on a second try I will start with a cut of the area and lift the mask off, since the smell and mess it leaves around it is quite bad.
And yeah, its a bit of trouble getting the right chem treatment so far locally.
Reduce the power, do a test burn on the mirror to get the results you want.
I have an Ortur2 and have etched the back off of mirrored surface with little frosting. I would have to look in my material library for setting but my max power is 15W.
An led type laser is at a different wavelength than the co2. The co2 will damage the glass while the led will pass through it…
I laser etch the back of the mirror so burn off the reflective material.
I understand this… if you burn off too much reflective material the laser passes through the glass with no damage to the glass.
A co2 shatters the glass once it has ‘removed’ the reflective material.
Can you directly etch the surface of clear glass with your laser?
This is what I found but it sounds tedious.
I paint the surface of clear glass with black tempura paint. It washes off with water. You only can do one pass.
You have to use something the laser can heat up and that heat damages the glass. With nothing on the surface you laser will not harm glass.
My understanding from TheLongMan’s original post is he wants to remove the mirror silvering without damaging (frosting) the glass (removed silver area is left as clear glass).
This means he needs to keep the laser from fully removing the silvering since any burning of the silver directly on the glass layer will cause the frosting from the heat.
Suggestions are to apply an acid resist to the mirrors silvered side and use the laser to remove just that resist layer, leaving an exposed surface to complete the silvering removal with chemicals designed to do that and not change the clarity of the glass.
Apologies in advance if I got this wrong in any manner! 
that is correct, and is the method that seems to work, for now.
Getting the actual correct chemical stuff to do it is a bit of a lark where I live but theres work being done on it.
I don’t have experience doing this or much else, but I did see a video some time back where the operator put the item he was working with in a pan of water and lasered it while it was inside the pan of water.
So you might try that.
interesting. unfortunately the sizes I am working on are much to damn big to do that effectively in any way.
Test it on a small one and see if it works. If it does, then it may be worth the effort to build a water pan for your machine. You probably won’t need anything more than 3/4 of an inch or an inch tall.
Water absorbs some frequencies in the ir range… Anything you ‘insert’ in the beam path will result in a loss to some degree.
I’ve tried it on my co2 and it was pretty much a wasted effort, IMHO…
Here’s a 15 second clip… There was little damage to the material, but I had warmer water… wonder where the energy went…? 
https://drive.google.com/file/d/19XpVXUL8bJGQxjwBRYhkHlh1LAgz77C8/view?usp=sharing
Doing 1 foot square mirror and using a jig, makes water idea pretty non functional.
Try it yourself…
It doesn’t matter what you use to protect the glass, the basic operation is to heat a small area, which causes it to shatter. This is how a laser damages glass. If it doesn’t shatter it won’t mark the glass. I don’t believe for a second you can ‘melt’ a small area and not have it shatter long before it reaches melting temperature.
Well I did some testing
The mirror I use are the inexpensive tiles sold by the big-box stores. Basically seem to be an aluminum reflective layer with a thin copper plating, then some form of protective paint (dark grey).
I tested the etching with an unmodified scrap of mirror. Muratic acid from the hardware store “pool cleaner” and drug store 3% hydrogen peroxide, mixed about 3:1 acid:peroxide. Let the mirror soak and in about an hour, all the backing material was removed.
I prepared some samples for the laser with various paints, all generally failed as the laser burned through the paint and backing with no clear way to control the depth of cut (of course)
I then ran a grid test with shelf liner. It claims “permanent” but could be easily peeled off.
The liner was smaller than my mirror sample (black area around the white liner), but this mirror sample had black spray paint covering the mirror backing and protected it perfectly.
There was a range of power/speed that seemed to burn away the liner before doing anything to the mirror backing. I’m assuming the adhesive was a suitable barrier.
After the laser:
I submerged in the acid bath with about 5 mm of solution above the mirror back.
30 minutes later I gave it a gentle brushing with an old tooth brush and could seee that the mirror backing was coming right off.
Not wanting to let any of the edges start to undercut, I pulled it out and rinsed with water.
It worked! 
I peeled the outer ring of liner off easily and I think there would be some mild solvent that would allow it to fall of without attacking the mirror backing, but leaving it doesn’t matter unless seeing the back side of the mirror is important
There was no undercutting, crisp lines and perfectly clear glass.
I should have let it soak for a few more minutes as the skinny line forming the edge of the turtle shell has some residue.
A quick check of Amazon listings suggests (all?) shelf liners are made from PVC film, which releases chlorine when cut with a laser. The fumes will corrode everything in sight, including your lungs.
Materials discussion:
Anything other than a chlorinated plastic will be a Good Idea™, although it may be hard to find.