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Repair Glazing Compound on Historic House or Building Windows | Wisconsin Historical Society

Guide or Instruction

Repairing Glazing Compound on Your Historic Windows

Repair Glazing Compound on Historic House or Building Windows | Wisconsin Historical Society

An inexpensive way to improve the energy efficiency of the windows in your historic house or building is to keep up on regular maintenance. Your window maintenance practices should include repairing the glazing compound that holds the glass window panes in their wood frames. The glazing compound seals the wood, helps hold the glass to the sash and sheds water away from the wood. Failed or missing glazing compound is a significant cause of energy loss in any window.

Choose the Right Glazing Compound

Before you install new glazing compound on any window, make certain you choose the right compound. Glazing compound comes in cans you can purchase in most paint stores, hardware stores and full service lumberyards. Always use a glazing compound with boiled linseed oil. This type of glazing compound will cure enough to paint within 24 hours. Beware of glazing compounds made with soybean oil. These compounds take up to 28 days to cure for painting.

Choose Replacement Glass

If any of your windows have broken or missing glass pieces, you will have to choose replacement glass. Double-strength glass is the best choice for matching the thickness and weight of the original glass. You can also ask local window replacement companies if they will give you some historic window sashes that they would usually take to the dump. These sashes have the wavy cylinder glass to match your old wavy glass. Have a glass shop cut it for you. Make sure the glass is 1/8-inch smaller in both directions to allow the glass to expand and contract as temperatures change.

Gather Your Tools and Supplies

To repair the glazing compound on your windows, you will need to gather the following tools and supplies:

  • Glazing compound with boiled linseed oil (not soybean oil)
  • Replacement window glass cut to size (if a glass pane is broken)
  • Putty knife, 1½ inches
  • Replacement glass panes for broken glass
  • Rubber gloves
  • Eye protection
  • Hammer
  • Sawhorses
  • Infrared paint removal device
  • Aluminum heat shield
  • Roller chisel
  • Carbide hand scraper
  • Brush
  • Boiled linseed oil
  • Denatured alcohol
  • Acrylic latex caulk containing silicone (not 100% silicone)
  • Razor blade
  • Rags

Step 1: Assess the Condition of Your Glazing Compound

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Step 1

Source: Bob Yapp

The first thing you should do is establish the current condition of the glazing compound on your windows or wood storms. Glazing compound is always on the exterior side of a windows sash. Many old and historic windows have a true divided light muntin. Divided light means multiple panes of glass in a window sash. These panes are in turn divided by wood muntins. If glazing compound was properly installed around the glass of your windows, it should last 30 years. This 30-year life depends almost entirely on how well the paint on the sash and the glazing compound has been maintained.

Many house and building owners remove only glazing compound that is visibly loose. If you have a lot of loose pieces, it won't be long before the rest of it comes loose. If this is your situation, remove all of the old compound and start from scratch. If you have just a few minor areas where the glazing compound is loose or cracked, you can remove those sections and re-glaze right up to the old compound.

Step 2: Remove the Window

EnlargeWindow repair

Step 2

Source: Bob Yapp

If you need to remove old glazing compound, the procedure will be quicker, safer and easier if you take the window sash completely out of its opening. If you have just one broken pane, you can remove the glass and reglaze the affected area while the window is in place. Depending on the type of window you have, removal can be simple or complex:

  • If you have a casement window, all you need to do is remove the hardware and hinges.
  • If you have a double-hung window, removal is more complex. Double-hung sashes are held in place by stops. Stops are wood strips that are secured to the window frame that prevent the sash from falling inward. Most stops are screwed (not nailed) into place, so you must remove the screws to remove the stop. Once you have removed the stop, you can pick the sash up slightly and tilt it inward enough to remove the sash cord from the sides of the sash frame. When the sash chords are removed, the sash should be free enough for you to completely remove it. Reinstall your double-hung windows by following these steps in the reverse order.

Step 3: Remove Broken Glass

EnlargeWindow repair

Step 3

See right hand side of photo where broken glass has been removed. Source: Bob Yapp

If you have a window with broken glass, wear gloves and eye protection to remove the broken glass. Remove the window sash and set the sash over a garbage can with the exterior, puttied side down. Use a hammer to carefully break the glass out. Most of the old glazing compound will fall out as you break the glass.

Step 4: Remove the Old Glazing Compound

EnlargeWindow repair

Step 4

Removing the glazing compound using an infrared heater. Source: Bob Yapp

It is usually best to set your windows up on sawhorses to remove the old glazing compound, but this task can be done with the window sash in place. Use a putty knife to remove all the loose glazing compound before moving to the next steps. Heat is the best and safest way to remove glazing compound. If you try to remove the compound without heat, you are likely to ruin the wood and break the wavy cylinder glass. If you use a heating device with a heat shield, you should lose only 5% of your old wavy cylinder glass. If you remove glazing compound cold using chisels and putty knives, you will probably lose 50% of the glass.

CAUTION: Do not use a blow dryer type of heat gun as a heating device. The highest setting on a blow dryer type of heat gun can generate heat of over 1,000 degrees Fahrenheit that can break glass as well as burn wood. Also, if your wood windows have lead paint on them, the high-temperature heat will cause a chemical reaction that emits a poisonous gas. This gas can harm or even kill you if you are working in an enclosed place. If you even consider using a blow dryer type of heat gun, it MUST have an adjustable heat setting. Do not set the heat gun higher than a medium heat setting.  

The safest heating system to use is an infrared heating device. An infrared heating device will not generate a high enough temperature to break glass, burn wood or produce lead vapor. An infrared device will cost several hundred dollars, but this will seem like a bargain if you consider how much faster and safer the device is to use compared to a heat gun. You might be able to rent an infrared heating device from a tool rental or paint supplier.

Set an aluminum heat shield over the glass to reflect the heat away. You can create a heat shield with 3" x 6" packs of aluminum flashings or by wrapping a piece of wood with aluminum foil. Set your heat shield over the glass next to the glazing compound.

Hold your heating device over the glazing compound and keep prying under it until the compound lifts up easily. Move the heating device away slowly. Use a roller chisel to remove the heated compound. Roller chisels have a roller bearing next to the sharp end of the chisel that helps you roll along the compound line and keep better control so you don't dig into the wood. If you do not have a roller chisel, you can use a putty knife or old chisel to lift the heated compound.

Step 5: Remove the Glazing Point

EnlargeWindow repair

Step 5

Source: Bob Yapp

Remove the small metal glazing point and the glass. Use a carbide hand scraper to scrape the remaining compound and dirt from the two-sided glazing bed.

Step 6: Prepare the Glazing Bed for Glass Reinstallation

EnlargeWindow repair

Step 6

Source: Bob Yapp

Create a 50/50 mixture of boiled linseed oil and denatured alcohol. Use rags to brush the mixture onto the wood. This mixture seals the dry wood and prevents the oils in the glazing compound and bedding caulk from being drawn into the wood.

CAUTION: Boiled linseed oil can be very dangerous if not handled correctly. A small rag that is soaked in linseed oil and lying in the middle of the floor can spontaneously burst into flames for no apparent reason. Follow the manufacturer's safety recommendations to dispose of rags soaked with linseed oil and to store any unused portions of the mixture.

Allow the applied linseed oil and denatured alcohol mixture to cure on the wood glazing bed for a few hours. While you wait for the boiled linseed oil to dry, you can clean the old or replacement glass.

Step 7: Prepare Glass Bed

EnlargeWindow repair

Step 7

Source: Bob Yapp

Apply a bead of paintable, acrylic latex caulk with silicone into the glazing bed. Be sure the caulk is not pure silicone, which cannot be painted. Set the glass into the caulk bedding and press it down until the caulk squeezes out on both sides of the glass. When the caulk dries, use a razor blade to remove the excess caulk.

Step 8: Install the Glazing Point

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Step 8

Source: Bob Yapp

Install the small metal glazing point that holds the glass firmly to the window sash.

Step 9: Apply New Glazing Compound

EnlargeWindow repair

Step 9

New glazing compound installed, ready for paint. Source: Bob Yapp

Warm a wad of glazing compound in your hand and push it into the glazing bed all the way around the window sash. Dip the tip of a very stiff 1½” putty knife into some boiled linseed oil and use the putty knife to smooth out the compound. Apply lots of pressure as you pull the putty knife along the rough bead of compound and remove the excess compound from the glass.

Step 10: Prime and Paint the Glazing Compound

EnlargeWindow repair

Step 10

Source: Bob Yapp

Allow the glazing compound to cure for 24 hours. Prime the compound with an oil-based primer. Apply oil or latex paint topcoats to your new work.

 

 

 

 

 

 

 

 

 

The information presented here is not intended to provide comprehensive technical advice or instructions on solving historic preservation issues. Any information contained or referenced is meant to provide a basic understanding of historic preservation practices. Read full disclaimer.