The original is probably Mica

Almost certainly. Find a ‘skipped’ electric iron?

You may have trouble trying to cut native mica into circles. I made a botch when I tried it, although it is done commercially. I suspect that a sharp circular punch might do the job.

Andrew.

James – Yes I would agree with others and say Mica too.

Not sure if this will be of use but recently on here I asked about cutting microscope glass. It was suggested by some that it may be able to be cut 'with a pair of scissors' under some water.

A quick google confirmed this is a method that could be adopted but in the end I didn't need to try it. Whether it would work with mica I have no idea but it might be worth thinking about.

Just a thought – Tug

If there is direct flame nearby, I would not recommend any polymeric material be used. There are some cyclic olefin polymers that will take 160 degrees C in service and some are flame retardant but none of them are rated for, or will stay clear, if exposed to flame. PTFE is opaque by the way, not clear/transparent.

Ordinary tempered glass as used in gas and wood fireplace doors would be my recommendation for use near direct flame, as it will take quite a lot of heat and will withstand rapid cooling from high heat as well, in case someone opens a door and lets a cold draught in. This material has to be cut before tempering. It will shatter if attempting a cut after tempering. Best thing to do would be to find pre-cut and tempered pieces the right size. It is usually not expensive.

Pyrex glass would also be excellent but it is much more expensive usually.

You may be able to find round gauge face glasses in Pyrex or tempered glass. If not, is there room to mount a square or rectangular glass piece in front of or behind the 60 mm opening, in a sheetmetal holding frame?

Glass will be much more durable than mica or composite mica board.

Just food for thought.

Edited By Jeff Dayman on 06/01/2021 16:44:15

Mica can be " cut " using a pair of sharp nippers – preferably of the end cutting variety. The type used in electronics would be suitable for this size of work. You need to nibble away of the edges until you reach the desired shape.

Pyrex would be suitable but would be difficult to cut to shape and ideally you should anneal the cut edge prior to use. A possible source, if you want to try it, is the base plate for a 3D printer. These can be acquired quite cheaply in the smaller sizes and possibly used uncut if it is possible to fit the square sheet behind the round hole.

Pero

Industrial boilers in small factories/hospitals, etc, have burner 'spyhole' glasses of about that size.

You could try your local British Steam Specialities BSS stockist.

Can you change the window viewing area with a metal plate fitted with a different size and shape glass, if so the best you can get is 'transparent fused silica' glasses which are available for example on eBay, just looking at one which is 37mm sq by 1mm thick, about £6 inc P & P, this type of glass can stand very high temperatures and thermal shock and for a window like this should be OK for 1000C see link below.

https://accuratus.com/fused.html

There is an interesting article about mica on the British Mica Company website, it will stand up to 900c, and the largest piece found was 330 tons!

Many years ago, as a country ironmonger we used to sell sheet mica for oil heaters and slow combustion stoves, the memory is fading, but I'm pretty sure we used to cut it with normal scissors. Happy days!