Cementing Leaded Panels

Cementing panels is as old as leaded glass - over 1,000 years - so it is a time-proven process using simple materials. The object of cementing is to make a leaded panel weather and water tight and sturdy. It can be messy and dusty, so putting on an apron and a dust mask are a good idea.

Part 1: Cement

You can make your own lead light cement as the materials are fairly common and safe to use.

Recipe

7 parts whiting/chalk

1 part boiled linseed oil

1 part mineral spirits (turpentine or other)

(All of the above measured by volume)

1-2 Tablespoons lamp black or other colorant – black poster paint, acrylic paint or oil paint

Tools and Materials for Making Cement

The mixer should be capable of mixing bread or similar stiff dough. The existence of a dough hook among the beaters is a good indication.  Also it is best if the bowl is rotated as this makes for much easier and more fully mixed cement.

Method

Add the whiting (reserving about one quarter) to the linseed oil and mineral spirits. Mix this well, by hand or with a domestic mixer capable of mixing bread dough. When these are mixed thoroughly, check the consistency. It should be like molasses on a cold morning - barely fluid.  At this point, add the colourant, so you will know the current colour and can adjust to make it darker.

Add more whiting as required to get the consistency you want. Experiment a little to find what suits you best. If you have to deliver the panel quickly, for example, you need to increase the proportion of mineral spirit. The mineral spirit evaporates relatively quickly, leaving a more rapidly stiff cement.

Being mixed

Note that the cement is nearly finished mixing as the ribbons of material are forming in the above photo.

Mixed and ready for application

When fully mixed the cement should only slowly drip off the mixer beaters.

Comment

You should make only what you will be using on the current project, as the whiting separates from the linseed oil and spirit relatively quickly. The commercial cements have emulsifiers to keep the whiting from settling and so extend the life of the product. Since making your own is cheap and quick to make, there is no saving in making a lot.

Part 2: The Start

Start on the side that is already facing up after soldering. This normally will be the rough side. This way you do not have to move the panel much until it has stiffened with the addition of the cement.

Starting with the rough side enables you to cover all open bubbles, rough glass (waffle, ice, etc.) and all painted glass with masking tape before you start. Put the tape over all the relevant areas of the panel, then use a sharp craft knife to cut the tape at the edges of the came. The cement will go under the came, but not into the texture of the glass. This will make the clean up of the glass much easier after cementing.

Adding the cement to the panel

With the panel on the bench, put a dollop of cement on the glass and rub it in all directions with a stiff, but not hard, bristle brush to force it under the lead. 

Scrubbing the cement under the leads

Adding whiting

When the cement has been pushed under all the cames, but with a slope of cement showing, spread a little fresh whiting or sawdust on the panel and gently push it against the cement under the leads. This begins the setting process and keeps the spreading cement from sticking hard to the glass or bench.

Adding cement to second side

Turn the panel over to cement the second side the same way as the first. If the panel is a large one, you may want to use a board to support it in these early turning stages. No gaps can be tolerated in the cementing. Cement leaking out the other side is good evidence that all the gaps between the glass and the came are filled. Again, after cementing, sprinkle new whiting/sawdust over the second cemented side and rub it gently into the exposed cement.

Scrubbing second side

Adding whiting

An illustration of the need to ensure the cement is well scrubbed under the leads comes from an occasion I had to repair a panel made by a client's friend several decades ago.  It was cemented by pushing commercial putty under the leaves of the leads.

This photo shows how the putty filled the space above and below the glass but not between the glass and the heart of the came.  It also shows the putty missing from the corners of the glass. 

The question may be asked about what is so important about a bit of putty missing from the edges of the glass, it is sealed along the leaves of the came. Yes, this style of cementing will seal the panel from the weather for a time. But had this glass been a window instead of hung inside, it is questionable as to how long it would have been until it began to leak - possibly only 20 years after being made. Certainly as the putty begins to break down, the moisture will rapidly find its way into the inside.

The only way to be certain that the panel is completely weather proofed is to use brushable cement. The cement is pushed under the leaves of the lead with a stiff brush. You know the fill is complete by the cement oozing out of the other side.

Part 3: Setting Up the Cement

After the pushing the cement under the cames on both sides, flip the panel over and begin a firm rubbing to push cement into the gaps between the lead and glass. Sprinkle the used dust from the bench top over the panel and rub in all directions. This begins to set up the cement by helping to provide a stiff skin over the more fluid cement. Brush until the whiting is largely off the panel. Turn the panel and do the same for the second side. Several applications of whiting/sawdust are required to give a sufficiently thick skin to reduce the amount of spreading, leaking or weeping cement.

Once both sides have been done a couple of times, begin to concentrate the brush strokes along the lead lines rather than across. This will begin the cleaning phase and also begin to darken the came. Repeat this on the other side.

After a few turnings, most of the cement will be cleaned from around the leads. Don’t try to get all of it away, you will need that colour for polishing. The glass will be shining, and any felt tip marks you made on the glass will have gone too. Clean up the dust from the panel and bench in preparation for polishing.

Part 4: Polishing Lead Cames

Use a soft brush to polish lead came. Don't pick out the cement until the polishing is done, as it provides the colour for darkening and polishing the lead and solder joints. 

Polishing with softer, finer bristled brush

The action with the polishing brush should be gentle and rapid, much like polishing shoes. If the shine does not come, you can use a very little stove blackening (carbon black mixed with a little oil) If you use a lot, you will have a big clean up job. A little stove blackening spreads a very long way.

Picked out, but not swept, polished inside

Before turning the panel a final time, put down paper or cloth, to avoid scratching the solder joints while polishing the other side. The result should be shiny a black came and solder joints that does not come off the way a final buffing with stove blackening does.

Finally, pick out any remaining cement.

Picked out outside

Resting, inside up

Rest horizontally with weather side down for traditional installations. If the panel is going into a double or secondary glazed unit, you may want to reverse this. The reason for having the smallest exposed cement line on the outside is to allow the water to run off the window with the minimum of area to collect. In a sealed unit or for secondary glazing, you may want to have the smallest amount of cement showing inward for appearances, as there is no weathering reason for the traditional method.

Rest for a day. Pick out the cement again. If the cement was stiff enough, there should be no need to do any more picking at the cement after this.

Brushes

Use stiff, but not hard bristle brushes for cementing. Nylon scrubbing brushes have a good stiffness without being too hard. Some natural bristle brushes are very hard and scratch the came excessively. In general, moderately stiff brushes with about 35 mm bristles are fine for cementing. As they do not last very long, they should be cheap, but with firmly attached bristle bunches.

Cleaning the brushes is very simple. The action of rubbing the cement under the leads with whiting causes a natural cleaning action to take place. As the bristles flex back and forward over the came, the cement is forced upward toward the handle, and then outward between the bristle bunches. Only a little effort is required to finish the cleaning: push a rounded stick between the bunches to move out the remaining cement. You now have a clean brush for the next job.

The alternative is keeping the brush in water, but this presents the problem of getting rid of the water (oil and water do not mix) before beginning to cement. As the water will emulsify with the linseed oil, it will be carried into the putty, leaving gaps in the cement when the water eventually evaporates. The cement will eventually harden, even though in water, as linseed oil cures by creating an organic polymer through oxidisation. It can also rot the wood handles.

Keeping the brush in mineral spirits does keep the brush flexible but requires drying/evaporating the spirit before beginning the cementing to avoid the residue of the spirit creating cement that is too thin at the start. This can be a really messy problem!

If you choose the “dry” method, it is important to keep the brushes free of hardened cement as it will scratch the leads badly, if not the glass also. Most brushes will only last 5-10 uses, and as they are not expensive, should be easy to throw away.

Polishing brushes should have more and finer bristles than cementing ones.  They can range from the very soft shoe polishing brushes to medium firm.  Many stained glass suppliers sell these, but a good shoe polishing brush kept clean will last for years.