Pat,
I replied to your email over on ArtAffairs.com re this, but basically cut & paste . . .
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Although a wall mount cutter may score the glass fine, the thickness may be a problem to run etc like normal picture framing glass.
Cutting on a bench is likely easier and safer.
1/4" mirror is different to thinner glasses.
You score the same, with a little light machine grade oil on the wheel to keep the cut fresh, and then you have a few options.
1/ With the correct fingers under / thumbs above glass running grip, you can run a good clean sharp cut on 1/4".
Depends too on how much offcut you have.
If enough, then a good firm running movement can work fine.
If it's only a couple of "'s you need to take off, you need to use glass running pliers. Normally the smallest size pliers like this
http://www.crlaurence.com/ProductPages/R/RD1932_772.html?Origin=
If that link doesn't work go to the CR Laurence website, search page, type in glass pliers.
The drop jaw pliers give good leverage with thicker glasses.
Takes a bit of practice, but easy to run 1" or so off 1/4" plate.
(You can feasibly run just a bit wider than the glass thickness off a length, but let's not get too ambitious !)
If your offcut is fairly wide, you can drage the scored glass up to the bench edge (a bench that has a soft cover etc, and nice straight edge) and drop snap it with the score about 2" onto the bench.
Another way to drop snap is to score, then lift the glass up, have an assistant slide a timber straight edge under the score, then you just drop it onto this.
The straight edge should be about several "'s wide x width of the sheet x about 1/2" to 3/4" thick.
Hope that helps, if you have any questions, feel free to post them here or e-mail me.
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With that in mind, a great idea has been posted in several replies by others . . . get ye to a glass shop.
They would cut these for you no problems, some at no charge for a bit of goodwill. Takes seconds a cut.
The other reason to go to a glass shop is they will likely have some form of edging machine to take the sharp edges off and give it a nice look.
A polished edge by auto machine is best, but an arrissed edge may suffice.
Usually bathroom mirrors are best fitted with the holes drilled and proper fixings (with little cover domes) as this method allows ventilation of moisture.
If siliconing to the wall, ensure you use neutral cure silicone, and brace them (or support with some good double sided tape) while the silicone cures, at least a few days.
(Be VERY careful if siliconing and the glass isn't supported on the vanity top etc itself, support with some sort of horizontal timber braced from the vanity top.)