On 19/06/2020 10:00, Smolley wrote:
I need to cut some glass 4.5mm (a mirror), do I need to score both sides
or just one side. Can I use washing up liquid for lubricant instead of
oil ?


Score only on the non-silvered side - ideally using a carbide-wheel
cutter against a straight-edge.

You need to keep the cutter vertical and apply the right amount of
pressure - you do not want to press so hard that you see glass chips
along the score-line.

Use any sort of oil - I use good old 3-in-1 oil, but any light oil (even
cooking oil) will do at a pinch.

Having scored the line (in one single confident stroke - do not go back
over the line) then tap firmly on the underside of the glass under the
score at both ends. You should see the score start to run though the glass.

Lay something (like a thin lath or even a thin metal rod) underneath the
score and apply downward pressure to the sides of the glass.
The crack should run along, under the score.

Mirror is very susceptible to the dreaded 'creeping black-edge' - where
the silvering detaches from the glass and the reflective effect is
marred by the black area.
Grinding the edge of a mirror makes this more likely to happen, as does
the use of any type of 'acid-cure' mastic.
Best to seal the edges with a cellulose varnish or clear nail-polish.

Thing is - it's a fairly specialised skill, and if you're only doing it
once then there's limited opportunities for developing that skill.

Any glass merchant would do it for you for a fiver - and get it right.
I'd do it - but you'd have to get it over to me in Ireland!

Adrian