From bitter personal experience Crittall = without doubt the worst window manufacturer I have ever had the misfortune in dealing with ! (several times)
Various versions of fly-ash such as GGBS (ground granulated blastfurnace slag) or PFA (pulverised fuel ash) are used in specific concrete mixes, they can give resistance to sulphates in dodgy ground and act as a cement 'replacement' making it a (bit) greener
So can you still get the various fly ash? When we were using lime mortar to brick up a redundant doorway, the specialists suppliers could supply all the other components but not the fly ash, and it was the specialists that identified the constituent components of our specific fly ash lime mortar. So we had to use blast grit for consistency in the look of the mortar to blend in with the surrounding brickwork. But we are looking to to undertake some lime mortar pointing and want to be consistent with the original...over time the colour of the mortar has changed from when new, the suppliers just slightly change the mix to match the lime mortar accordingly.
Never dealt with Crittall windows, although they were the original suppliers to our house in the 1924. We used a different company as we wanted the same look but double glazed and better weather protected.
Its always interesting to see the design layout priorities in the different individual houses in our street, next door and next door but one ( owned for a time by Bobby Ball the comedian) are relatively much smaller houses but have massive entertaining/drawing rooms, Ours by comparison had a massive billard room, but much smaller entertaining rooms...................maybe they only enjoyed the company of a few people compared to the other house owners. Strange old world, Speakman our original owner whilst the poorer relation (relatively speaking) of a Lancashire coal mining dynasty, was a mine engineer who made his separate fortune from making engineering bricks from coal dust. Whilst renovating the original kitchen in the downstairs servants quarters, stripping back 70's anaglypta, we found hand blocked Chinese painted figures/scenes against a very dark brown background on the wall. Unfortunately, less caring electricians in the 50's/60's had run channelled wires thru the scene. The size of skirting boards, the thickness of doors, depth of plastering on the walls varied from the owners to servants quarters, which meant all the servants quarters woodwork etc had to be made consistent with the rest of house. Although a summer house and never used outside the summer season, it has massive coal fireplaces and 24 double sized ventilation bricks around the house to allow adequate ventilation and draw for what must have been roaring fires in the house, that's beside the basement furnace system. The house was split into 4 flats in the mid 50's as there was little demand for large houses then, we own 2 of the flats one downstairs and one upstairs, we have thought about buying the other 2 flats and bringing the house back to one, but the thought these days of the cost trying to heat the entire house isn't worth thinking about and I have better things to spend my money on and I don't have access to an unlimited supply of coal as Speakman used to enjoy. .