Architectural and Restoration Glazing
Glazing has now got better and better through the years eventually ending up with float glass or annealed glass. Invented by sir Alistair Pilkington in the 1950s of Pilkingtons Glass. Moulton glass is poured into a tin bath and levels out as it spreads out and levels along the edges giving a brilliant flat finish on both sides. As the glass cools and solidifies it leaves the bath in a continuous long thin slab. The glass is then annealed by cooling in a Lehr “oven” and then the finished glass plate is finished perfectly parallel.
Pilkingtons Glass is still one of the biggest manufacturers in the UK and they are constantly coming out with different products. One that is catching the eye of a lot of people is the Pilkingtons Spacia glazing. This is only 6mm thick and gives the same U-value of 28mm double glazing, but the downside of this is it is made in Japan and is about 9 weeks from order to delivery, rather than 5 days with Slimlite Glazing please see our dedicated pages on Pilkingtons Spacia and Slimlite Glazing.
Cylinder and Crown Glass
Float glass is not the same as the glass that was originally in your sash windows. The original glass was called Crown glass and then it changed to cylinder glass which is still made in 3 factories in Europe – Jaslo in Poland, Lamberts in Germany and St Just in France. When the industrial revolution came, depleted forests in Europe changed the way that glass manufacturing was done. From the original charcoal furnaces people had to swap to a more reliable source of heating the raw material and that was to coal and then gas.
Glass used for glazing is made by heating together sand (silica) 70-74% and lime (calcium oxide) 5-12%, at a high temperature with a flux of wood ash or soda 12-16% to reduce the temperature at which the particles fuse.
Why Cylinder glass has different colour tints
The sand was taken from anywhere they could get it including quarries and rivers with not much regard for any contaminates that could have been in the river, this is why old crown and cylinder glass was rarely white or neutral. Sand often contained metal oxides, used in the smelting process. Beachwood ash and soda lime used in the smelting process also gave it a different colour which was mostly green or straw coloured, you will find more crown glass with this sort of tint as it had less tax on it in the mid-18th century this was because it was taxed on the weight of the finished product.
When regular sources of materials came more available in the 19th century and you had more efficient coal furnaces it was more common to have white glass produced. This was more favoured with cylinder because crown glass seemed to diminish due to the fact that it was far inferior a product than cylinder even though it was still made side by side upto the 20th century.
How Crown glass was made
Crown glass was the easiest of glass production with it being dipped in the mouton glass and mouth blown to a bubble, cut open and spun on a flat table until it was in a rectangle shape. This is why you would mainly see crown glass in leaded windows because you could only really get 3 small lights out of one sheet of crown glass. This procedure created swirls in the glass and bubbles this is one of the reasons that crown ceased to exist any-more. glass was such a expensive product at the time that nothing was wasted. The excess of the glass that was cut off was called a Bulls Eye and was ground down flat and probably bartered away for produce or ale and you can see this in many old pubs and buildings that was used for bakeries etc around the 17th and 18th century.
How Cylinder glass is made
Cylinder glass is handmade, dipped in the fused molten glass mouth blown into a cylinder bell shape, people would swing it from side to side to make longer in a pit. Then it was cut off at both ends, then in the middle, and put in a furnace to straighten out. This is what you will see in most stained glass products because it was easier to mix the products and had less waste. Even though cylinder is still distorted in a elongated way, it was a little easier to produce.
Handmade glass brings vibrancy and beauty into your property, the way it disperses the light and gives that warm feel with a subtle tint that gives it a certain vitality. In the same way you wouldn’t use modern day bricks in a Victorian or regency house. The new float glass of today would look stagnant and soulless pretty much like the bricks that we explained before. Cylinder glass is really the only option if you want that authentic antique look in your windows from yesteryear.
Even when using Cylinder glass you have to be very careful of selecting the right type of cylinder glass. This has now evolved into more than 4 different categories from Antique, Russian, Reamy or Seedy Cylinder glasses produced for the crafts or Art market and this makes them poor substitutes for historic window glass.
One of the best ways to see the difference is to look around many of the Victorian properties that we have in our great cities, and look at the reflection of the glass. You will notice the difference when someone has changed the glass opposite cylinder glass with the plain flat look of float glass like a soulless picture.
P1 Restoration Glass
This is a cheaper option with the glass being heat treated to give it that wavy distortion look of cylinder glass. This is an option that is on supply in the UK and hasn’t got a long waiting list. This is a very good compromise we have found for cylinder glazing with it subtle shades as well as being able to duplicate the colour that you get in older crown glass. This is also found in Architectural Slimlite double glazing as well. Please see our dedicated page on Slimlite double glazing and how it can transform your sash windows if you have any questions please contact us on the form below.