Let's start with 'hermetically' sealed units. Good word, isn't it? Sounds very impressive and a 'must have'. This description is often used by manufacturers who use the 'hot-melt' method because it is efficient to make and when made it can be handled and used almost straight away.
Glass bonded together using either 'two-part polysulphide' or two-part polyurethane, although technically still 'hermetically sealed', is the product of a more laboured manufacturing process and may need to be laid flat to cure, I believe, for some 20 hours.
Another type with a very silly name, 'swiggle strip', is manufactured in Wigan by Tremco Ltd. This consists of an 'all-in-one' sealant desiccant and spacer bar that heat-seals the two pieces of glass together as it is run through a hot rolling machine. I myself wish such machines were calibrated for several different recognised sealed-unit thicknesses rather than allowing the operator to make a coarse adjustment, which can lead to possible unintended odd thickness units. Also, I don't like the black colour, which attracts heat from the Sun, particularly if the unit is not very deep into the glazing rebate.
With leaded-light sealed units, real lead strip (3mm, 6mm or 9mm wide and in either a flat or an oval finish) is normally fixed to both sides of the outside pane. One single pane is used rather than separate pieces of glass because these would leak and then could not be truly hermetically sealed. Real lead exposed to the atmosphere for the first time will need time to oxidise and form a darkened coating. Until this happens you may expect some running marks on the glass, especially showing up when it rains. This is quite normal. Some manufacturers wipe the lead with a 'patination fluid’, which can be obtained at DIY stores, but vinegar works just as well.
All about 'misted-up' sealed units
Do sealed units have a vacuum?
Types of glass
About safety glass
Air gaps and insulation
Why did my sealed unit explode?
Brewster's Fringes
