Handle glass with care and wear protective gloves. Remember, that the edges may be razor sharp. Wear safety goggles whenever ther may be a risk of flying debris
Measuring up for the new window pane
Measure the height and width of the opening and deduct 4mm from each measurement. The glass should fit easily into the frame without catching at the edges. Frames are also rarely square so check the dimensions in several places.
Buying the glass and putty
Take these measurements to your local glass merchant who will also advise you on the appropriate thickness and type of glass.
On older properties, you may find that the glass has to be a little thicker than the original as standards and rules governing this may have changed. These rules relate particularly to permissible sizes and thicknesses for safety reasons.
You should also buy sprigs and putty to suit the frame. Again, your glass merchant will advise on the type of putty required. There are different types for wood and metal frames.
Applying the back putty
Remove the putty from the container and knead it like dough to make it workable. Often, the linseed oil in it will have separated out a little so take time to work it back in. It can be very sticky and difficult to remove from your hands.
When washing, use washing up liquid first – this will blend with the oil enabling it to be washed away with water
If the putty is still too oily after kneading, a good way of removing the excess is to squeeze it in old newspaper. This will absorb some of the excess making it easier to work with. Old, rather dry putty, can be rejuvenated by adding linseed oil to a hollow formed in the putty and working it in gradually.
Now take a tennis ball sized piece of putty and, holding it in the palm of your hand, feed it out into the frame rebate using your forefinger and thumb. This will form the backing putty against which the glass sits. It should form a continuous strip a few mm thick.
Fitting the glass
Take the new glass and place the bottom edge into the putty at the base of the rebate. Push the glass gently back into place and, by applying light pressure at the perimeter, squash it against the putty. Do not apply too much pressure as this may crack the glass.
Glazing sprigs or brads
Use the glazing sprigs to secure the pane. These are held flat against the glass and tapped gently into the wood frame with a small hammer. Lay the hammer sideways on the glass and use a sliding action to drive in the sprigs. If the hammer is kept in contact with the glass, possible breakage will be avoided. The sprigs should be behind the eventual line of view through the glass, but not so far in as to be too close to the edge of the pane causing it to crack. With metal windows, refit the spring clips in the holes in the frame to secure the glass.
Applying and shaping the front putty
Take another ball of putty and feed a band around the perimeter of the frame.
Starting at the top, shape the putty to a chamfered line using a putty knife. The action is that of drawing it along the putty and squeezing it at the same time. The process may require several passes of the knife to get it right and to remove the excess.
A useful tip is to dip the knife in a little water before the final smoothing to prevent it from dragging the putty.
At the corners, finish the putty with a mitre using the shaped part of the knife.
Once the top is shaped, continue with the two sides. Finally shape the bottom putty.
Take time to practice a little and you’ll soon get the feel of getting it just right first time.
Remove the trimmings with the point of the knife.
Remove the excess back putty from the inside. This is best done by running the point of the knife at right angles around the perimeter. Then, use a gentle angled pushing action to remove the offcuts.
Allow the putty to harden for about three weeks before painting to match the original decoration. Do not leave it untreated as it will dry out too much and crack.
Clean the putty marks off the glass before they dry out using scrunched up newspaper but be careful not to damage the new putty. Alternatively, methylated spirit can be used on drier putty marks.