Clear window glass is so common in historic structures, we really don’t think about it. But window glass can be a character-defining feature, depending on its texture, color, and configuration. Sometimes even an inscription can be a connection to its past. This page acts as a comprehensive guide to window glass. It includes better ways of understanding original glass, easy tips to help you figure out what kind of glass you are dealing with, and most importantly, ways to best take care of it. This page is divided into two parts: history and maintenance.
History: Identifying What You’re Working With
Clear glass is most easily identified through technique and time period, so knowing the history can really come in handy. Listed below are 5 of the most common types of historical glass found throughout the years. For more information, read a great article written by Ben Sinclair on the history of window glass.
Broad Glass & Early Cylinder Glass
Originating in the 11th century, these glass types are the most traditional. Broad glass is made with a blowpipe that forms the glass into a cylinder, which is later cut in half with iron tools. It frequently bubbles and has lots of imperfections including corrosion. Cylinder glass is a similar process, except instead of iron, the glass is cut with diamond. Lamberts Glas has a great video that shows the specifics of making cylinder glass. In case a full repair is necessary, cylinder glass can be used to replace either type.
Crown glass is distinguished by its smooth finish on both sides. Crown glass was at its peak in the 18th century and is no longer in production. If your crown glass needs to be replaced, your best bet is to use cylinder glass.
Mass Produced Sheet Glass
Mass produced sheet glass was created using two methods: the drawn cylinder method (invented in 1830) and the Fourcault method (invented in the 1900s). In the 19th and 20th centuries, sheet glass was by far the most common. A good way to tell if you’re working with sheet glass is to look for a subtle wave and a lot of surface movement. If you find that your mass produced sheet glass is completely ruined, cylinder glass works as a good replacement.
Machine Rolled Glass
Also known as cathedral glass, machine rolled glass originated in 1888. It is a very textured glass, and there was a lot of variation in pattern and color of the glass over the years. This can make replacements almost impossible to find, but there are a lot of repair methods listed under maintenance that can help with preservation.
Also known as Norman slab glass, this glass is cut from rectangular molds. While it is possible to commission new slab glass, cylinder glass might work for you as well.
Maintenance: Conservation & Restoration
There are four conservation and repair options listed below. A lot of these techniques are ones you may already be familiar with, but I’ll briefly discuss the differences between them. BuildingConservation.com has an extensive guide that provides more information.
Resin Edge Bonds
While edge bonding with epoxy is often helpful with this type of maintenance, it will not help you in glass repair. Instead, situ resin edge bonding should be used. It is more fragile and works better with the glass overall. If you’re working on sash windows with crown glass, beautiful glass that isn’t falling apart but does have a lot of cracks, or glass that needs a quick temporary repair, resin edge bonding is your best bet.
Repair leads or strap leads can be really helpful when conserving glass. Strap leads require the use of a soldering iron to put the leads into place. When making these repairs you need to decide if you are using strap leads on one or both sides of the glass. If the glass needs stabilizing, doing a double sided repair would work best. If the crack is more minor, you should only need to repair the side of the window that faces outside.
Copper Foil Repairs
This method was originally used by the Tiffany stained glass company in 1880 and is often referred to as the Tiffany Method. While it was originally intended to be used on stained glass, copper foil repairs have proven to be much more useful for glass with plain glazing. Tin copper sheets are attached to both sides of the glass pane, trimmed, and then connected. The sheets are so thin that altering the original glass to fit the modifications will never be necessary.
Releading requires a lot of precision and a good eye. The heart depth should always match the heart depth of the original glass. It is important that if you come across historic lead you do your best to conserve it. If that isn’t possible, documenting the historic lead is still a good thing to do.