More lessons from the past:

I first learned how, in the early years of double glazing, many suppliers simply didn't understand much about it.

Newton's rings
One of my first large sealed units for a patio door developed a patch of coloured interference fringes that I knew were the result of the two nearly perfectly flat sheets of glass almost touching in the centre of the DGU. The spacer bar of this 20mm sealed unit was 12mm wide, so this showed there was something very wrong. My complaint brought the glazier's rep hot-foot to deal with it.  I was more than a little shocked to see him simply hammer a nail through the edge seal, put his mouth over the hole and blow hard into the DGU, filling it with his moist warm breath. He then used the head of the nail to squash the edge seal into the hole to "seal" it.
"That'll be OK now", says he, without any apparent understanding of how he had greatly reduced the life-expectancy of the desiccant, virtually guaranteeing premature unit failure.

Another supplier I visited on a rainy hot and wet summer's day had all the doors and windows wide open and the lids off half a dozen drums of desiccant, with no understanding of how the stuff works and the damage they were doing to it.

One of the sons of that supplier set up his own manufacturing business, but sadly, without proper glass-cleaning equipment, the units I bought off him had many finger-prints on the glass sealed inside, so that enterprise didn't go far.

Low-Emissivity coated glass

The first real technical development in this field was Hard Coat Low-E from Pilkingtons.   Originally called Kappa-float, later changed to K-Glass, the reflective Low-E surface film is sputtered on while the glass is still red-hot, and baked in. It's easy to handle and doesn't deteriorate in the stock-room, so is convenient for small businesses with fluctuating sales. Sadly, I found it brought complaints from customers about a blotchy appearance if the light was at a shallow angle. After a few expensive complaints I very soon stopped selling it.  

It was not long after, that a better alternative called Soft Coat Low-E appeared. This is a metallic silver coating applied by vapour-deposition onto cold glass. The thin silver film that reflects heat back into the room is virtually invisible when applied, but silver tarnishes when exposed to air, so this clever material has to be stored as two sheets with the layers of coated glass tightly packed face-to face to exclude air and prevent the tarnish. Once separated, the glass has to be turned into sealed units very quickly to halt the colour-change. So it's only suitable for use by large users who can cope with the big production volumes necessary to minimise wastage.

This once caused my business a series of expensive disasters not of my making:

Jay Webb - then leader of the Fenestration Associates -  was a good friend and Champion of mine, and I had great respect for his knowledge of the industry. So when he came to me with a large project he was overseeing near London, of course I was pleased to meet his every wish.

He wanted to use sealed units from a company recently taken over by one of his industry compatriots. The DGUs duly arrived and my staff started to assemble and glaze these factory-glazed cassettes. After a few hours, the foreman brought one to show me, with hand-prints all over it.
Blow me down! These DGUs all had the silver Low-E coating on the OUTSIDE, and were all tarnishing before our very eyes, illuminating the hand-prints of my staff.

Stop the job, explain the problem to Jay, then wait a week for the new DGUs.

Start again...
After a few hours, the foreman brings one to show me, and this time it is TOO THICK on one edge. It's 24mm instead of 20mm. How can that be?

Not difficult to see why. It was coming apart.  Our glass specialist clearly didn't know that the silver Low-E coating had to be removed at the perimeter so that the glass could stick to the spacer-bar.

Stop the job for the second time, and again await the delivery of replacement glass.

At last, only at the third attempt, did our specialist glass supplier get it right.

Who paid for the disruption and the wasted work?

You guess.

(in retrospect I should have billed the supplier, but he supplied Jay, not me, and you don't sue your friends. Besides, it was a busy time, and I probably had bigger things on my mind.)

Updated Nov 2018