Another new addict

[Triggaaar]

Your brick will go a lot farther if cutting into thirds with a wet saw is possible.

Looks like £100 to hire a proper masonry saw. Or I can use my 4" angle grinder

It's difficult to articulate, but I'll try -
With regard to thermal mass, based on the three ovens I have assisted in the building and running of, I do not think there is much difference at all in the time taken to get measurable pizza temperature on the surface.

Remembering that the bricks have a thermal conductivity, you get the surface hot, then the heat wicks away at a slower pace to heat the rest of the brick.

Indeed. Presumably they're not radiating heat fully until the whole brick has reached temperature? If correct, a larger brick won't be radiating as much heat as early, and you want that radiated heat from the dome for your pizza.

This is the way it seems to go with an insulated oven that is quickly brought to temperature for a midweek pizza for the family, but has not been saturated.
When you do wish to heat to saturation, thinner dome will require less fire to saturate.

Makes sense. I guess it stands to reason that a smaller oven will also therefore need less fuel to heat up.

When the oven is heated to saturation, i.e. the bricks are as close to evenly heated all the way through as possible, then the extra thermal mass of a thick dome comes into its own.
Then the surface temperature of the brick as required for a good pizza recovers quickly between pizzas, etc. even if the live fire isn't very live.

A bonus, but not something I imagine is a problem in an oven with slightly less thermal mass.

The big thing with thermal mass is for retained heat baking. There is only so much heat you can store in a given mass that you heat with a fire in the dome.

Understood.

I much admire the FB Artigiano oven core. It's only three inches thick with the outer layer of refractory over the brick, and the FB plans say half a firebrick thick is a high but acceptable thermal mass for a residential oven. If you aren't planning on cooking for three days from a single firing three inches is going to be plenty.

Lol, no, I can't ever envisage cooking for more than a day, maybe just half a day.

I take it the 3 inch v 4.5 inch is the same price if you are buying pre-cut?

Assuming a full size brick is 230 x 115 x 65 mm, they're basically twice the price of a 3" brick (75 x 115 x 65 mm). So I could cut the full brick in half and they'd be 4.5" and the same price as the 3". As Gulf says though, I could cut them into thirds and it would be cheaper. I'll want to be doing cuts anyway so that the bricks join reasonably well on the inside. The fuel requirement is possibly more of a concern than the cost of the brick.

The longest I can imagine wanting stable temps inside the oven is something like 12 hours, for a Boston Butt or something. I've no idea how that would go in a 3" or 4.5" oven.

[wotavidone]

Looks to me like the plan is set.
36 inch oven, 3 inch thick walls, serious insulation. Can't go wrong with that.
Now - how to cut the bricks.
Well - I went to a garage sale one day and bought one of these for $20 Australian.
It was a piece of crap but I replaced the screws that held the main bearing carrier in, had to tap a couple holes to a larger size if I recall,and put in a masonry rated cutting disk.
I soaked the bricks and used a squirt bottle while cutting.
It's absolutely cactus cafucteye now, but it did three ovens.

[Triggaaar]

Looks to me like the plan is set.
36 inch oven, 3 inch thick walls, serious insulation. Can't go wrong with that.

Sounds good.

Now - how to cut the bricks.
Well - I went to a garage sale one day and bought one of these for $20 Australian.

You got lucky. I did some quick searching online, and found a wet stone cutter for £70 (would be cheaper used), but it only cuts 30mm depth, which isn't much good. By coincidence it's the same brand you picked up:
http://www.gmctools.com/en-GB/Saws/Wetstone%20Cutter

I'm struggling to find one like yours, I assume it's discontinued.

[wotavidone]

The one I bought was a 355mm/14" dry saw, called a cut off saw here in Oz. Normally used for cutting metal with an abrasive blade, but masonry blades of the right diameter and speed rating can be bought at the hardware shop. I had a quick search on ebay.co.uk and found that the saws are still available new in the UK, but prices start at about 99 quid and go up to 500 quid depending on brand. Getting a bit expensive.
In any case cutting masonry on them fills them full of dust and they must be considered a throwaway item. Hence I bought a cheapo secondhand one that needed some repairs.

An acquaintance used the same sort of saw, but actually fitted a water supply to make it a wet saw. I didn't trust my wiring at the time (didn't have an residual current device to protect me from electric shocks at the time) so I wasn't game to convert mine to a wet saw.
That saw you linked to would work fine if you have a bench you can clamp things to, like one of those Black and Decker Workmates. A 30 mm cut through a 50mm brick should ensure the brick will snap clean where you want it to.

[Triggaaar]

The one I bought was a 355mm/14" dry saw

Oops, didn't realise that. Is it much difference to a wood mitre saw? We can get cheap ones for £50 (plus brick blade):
https://www.screwfix.com/p/titan-ttb...230-240v/1295p
I assumed they're not an option?

Maybe like this?
https://www.youtube.com/watch?v=g55PNIajTEg

But then people say don't do it:
https://community.screwfix.com/threa...hop-saw.10734/

That saw you linked to would work fine if you have a bench you can clamp things to, like one of those Black and Decker Workmates. A 30 mm cut through a 50mm brick should ensure the brick will snap clean where you want it to.

Yeah I have a workmate. If I'm just going to cut part way either side, and then snap it, I might as well just use my 4" angle grinder? That's fine if I want to cut a full size brick in half, giving a 4.5" depth, but not if I want my cuts to be on the inside of the oven, which I assume I'd need if I'm trying to have a 3" depth?

[wotavidone]

Chop saw is just a basic heavy duty mitre saw I guess. Need to make sure you get one with a metal blade guard, not plastic. Add some sort of masonry blade. Check that you can actually fit the bricks in place to cut them, and there is room to clamp them in. DO NOT hold them with your hand.
Whatever you buy, it stands a good chance of being cactus afterward, and you might find it doesn't do the job for some reason, so consider buying used.