The process of waterproofing a leaded glass panel is a time-proven process using simple raw materials formulated as leaded light cement. In this blog I will consider several formulations for leaded light cement and discuss the various constituents that should and should not be part of the formulations.
The object of cementing a stained glass panel is to make the leaded panel weatherproof, to support the glass within the lead framework and to strengthen the whole structure in a way that makes it rigid and sturdy. When done properly the result is a leaded glass panel that could remain strong and weatherproof for more than a hundred years.
Let’s get cracking. There’s lots of chatter to get through and although some of it’s not going to be exciting I can at least promise to try and cover the subject in reasonable depth.
As an aside I apologise for not doing much chattering through this blog recently and observe with some wry amusement that much of the material for this blog was accumulated back in 2011 – meaning that many of the web references I found back then are long since gone. Oh, how ephemeral this Internet stuff can be!
Cement or Putty?
My first task is to explain why this blog is about Leaded Light Cement and why I do not call it Putty. Using the right word is important because words have meanings.
The words “putty” and “cement” are not interchangeable because there’s a real difference in what they mean, what they are, how they behave and how they are used. Put simply, we use a putty to hold glass in a wooden window frame but we use a cement to waterproof and bond glass into a leaded light window panel.
As the Wikipedia entry for Putty explains, putty is a generic term that describes highly plastic materials, similar in texture to clay or dough, typically used in domestic construction and repairs as a sealant or filler. Notice the words “sealant and filler” do not imply any sense of binding or adherence.
By contrast Wikipedia’s entry for Cement describes a substance that sets and hardens and adheres to other materials so that it can bind them together.
Leaded light cement is therefore a product that we expect to bind to the glass and lead work and we can also expect it to harden as part of a setting process. By contrast putty does neither. This subtle difference explains why “putty” and “cement” are not interchangeable terms even though you’ll soon realise they both have a formulations substantially based on whiting and linseed oil in our sphere of interest.
As I’ve just mentioned, putty and leaded light cement both contain whiting and linseed oil but that’s as far as the similarity runs. So, it’s the additional materials you will find leaded light cement that changes it from being a putty, causing it become a cement. You will learn more about these differences as you wade through my seemingly endless chattering.
Later you will begin to understand why materials such as Plaster of Paris and Portland Cement should never be part of any formulation for putty or leaded light cement.
What Is Glazing Putty?
Although this chatter should be about leaded light cement, it does no harm to briefly mention glazing putty if only to understand more about how it simultaneously has similarities and yet differs from leaded light cement. I’ve already given you some outline hints. But as usual I have more chatter to complete the picture.
Traditional glazing putty, also known as Common Putty, is whiting and linseed oil mixed in a proportion that forms a firm dough that is neither too hard nor too runny to glaze windows. This is just one kind of putty described at Wikipedia. Later in this blog I will look more closely at Whiting and at Linseed Oil as ingredients.
Although I have occasionally encountered comments suggesting leaded lights can be waterproofed with glazing putty, a common complaint is that it is not blackened and that it is the wrong consistency to force under the leaves of lead cames.
A further problem can be seen when comparing old windows in-situ. Leaded light cement deteriorates far more slowly than glazing putty. Leaded light cement is expected to last a hundred years whereas glazing putty will typically last tens of years.
In summary it’s clear that you shouldn’t be using ordinary glazing putty to waterproof a leaded light panel.
With a clear idea of what common glazing putty is, let’s have a look at some leaded light cement recipes. To start with I will generalise. Then we will look at published recipes. We will discover there are many variations on the same theme, some good, some bad and some to be avoided.
Leaded Light Cement – Generalisations
There are numerous recipes for leaded light cement, both proprietary and published, but the core common ingredients are always the same – linseed oil and whiting – so a similarity with traditional glazing putty is not in dispute. However, beyond these two core ingredients are other materials that make it more suitable for waterproofing and strengthening leaded lights. Principle amongst the additional materials are a volatile drying agent and a blackening colourant – more about these additions below.
The consistency of leaded light cement is a matter of personal preference, but should be firm enough that it can be forced under the leaves of lead cames without running through to the back. So, although leaded light cement could be as stiff as Plasticine or Play-Doh, which tends more towards the consistency of glazing putty, it has a more useful consistency when it is less firm than traditional glazing putty, variously described as peanut butter, porridge, a very thick pancake batter which slowly oozes off your hand, treacle or like molasses on a cold morning.
Some published formulations use only boiled linseed oil and others only raw linseed oil. There are others that use half boiled linseed oil and half raw linseed oil. Decisions about which to use or in what proportion relates to their drying properties, more about which I will chatter about later. For the moment, just remember that raw linseed oil takes a longer to dry and stiffen up in leaded light cement than boiled (or refined) linseed oil. These two oils are known as “drying oils” and the word “drying” isn’t in the sense that we’d normally consider to be “drying”. It is very much an “industrial” meaning for the word which actually relates to a hardening process that is the result of chemical oxidation.
Turpentine was traditionally used as a “volatile drying agent” but it is now an expensive product for which turpentine substitute is a more cost-effective alternative. A more modern synthetic alternative that is equally effective but not as pungent as turpentine is spirit white, also known as mineral spirit. Again the word “drying” isn’t quite what we mean in ordinary conversation. The word “volatile” hints that we’re dealing with a substance that evaporates quickly.
The reason for also using a “volatile” drying agent (such as turpentine) in leaded light cement to evaporate quickly from the cement causing it to thicken more quickly than would normally occur with just linseed oil. It therefore accelerates the drying process. This is why leaded light cement hardens within a day or so but common putty (which has no turpentine or equivalent) takes many days to harden.
The primary reason for adding extra liquid drying agents to the cement is to thin the cement to less-firm consistency, so that the cement can be brushed more easily under leaves of lead cames. A less desirable consequence of too much of the liquid components is that it can mean the cement runs out of the back of the panel being waterproofed and remains soft for several days unless measures are taken to assist the drying and stiffening process. The volatility of turpentine, or one of its alternatives, will always results in a faster drying time when compared with an equally runny formulation that uses only linseed oils (whether raw, boiled or in combination).
Although the use of volatile drying agents, such as white spirit, will aid rapid drying, the proportion used should not overwhelm or replace the linseed oil component because it is the long-term polymerisation of the linseed oil that is important to the end result. It is a matter of conjecture that the use of too much volatile drying agent may cause shrinkage or increase porosity of the cement in a leaded light panel.
Leaded light cement typically contains a black colourant which traditionally would be lampblack (black carbon powder, soot) but modern alternatives that are equally effective are “Universal colorants” (powder or liquid), concrete colouring powders, oil-based paints, powdered poster paint or stove blackening compounds. The resulting cement is a medium grey colour that approximates the colour of weathered lead. Be aware that the use of water-based pigments (such as ready-mixed poster paint, acrylic colours or latex) will cause problems because linseed oil and water don’t mix easily.
Some leaded light cement formulations contain small amounts of lead and lead compounds such as white lead, red lead, or litharge. Although no clear references have be found to explain why these lead compounds are sometimes used in leaded light cement formulations, it is reasonable to admit three plausible reasons, any or all of which may have explain their historic use:
- Lead compounds in the cement may impede deterioration by biological agents (such as fungal or microbial attack) due to lead toxicity. Comparing the longevity of leaded light cement in comparison with typical traditional window glazing putty supports this possibility.
- The inclusion of lead compounds may lend to the cement a colour and general appearance more reminiscent of lead came than the use of a black colourant alone. It is plausible to suggest that the long-term oxidation of lead compound inclusions in the cement will mirror long-term oxidation of the lead came of a leaded light.
- The lead salts might be used as a “metal drier” or an anti-oxidant. The use of red lead has been noted as a “hardener” in a recipe from the start of the 20th century and white lead has been used in the past in paints.
In addition to first noting that all lead compounds are toxic, outline notes about common lead compounds is perhaps useful, if only for future reference:
- White lead has the chemical formula (PbCO3)2Pb(OH)2 and is the pigment used in old paints and old cosmetics. It is now largely banned due to the lead poisoning it causes.
- Red lead has the chemical name lead tetroxide and has the chemical formula Pb3O4 (or more strictly 2PbO·PbO2) and may become yellow lead following exposure to damp air over time (as the monohydrate).
- Litharge is the red form of lead(II) oxide which has the chemical formula PbO. The other form of lead (II) oxide is yellow and is called massicot.
- Lead acetates also exist, and may be produced as a white bloom on new lead cames after a leaded light has been installed using modern synthetic grouts that emit acetic acid vapours when they cure.
With a foundation understanding of the main constituents found in leaded light cement under your belt, it’s now time to consider a few of the many published formulations for leaded light cement.
Some of the recipes below are eminently suitable whilst others contain questionable materials such as Plaster of Paris or Portland Cement. After each recipe I will add some comments to give you a deeper understanding of what I think is good and bad about each one of them. You, of course, are entitled to do your own research, come to different conclusions and disagree entirely with my opinions.
Recipe 1
This is a simplified traditional style formulation. It is the essence of what I read at Ehow.
Method:
- Measure boiled and raw linseed oil in equal parts (for example, 1/2 cup of each) into a bucket.
- Mix in whiting a cup at a time with a spoon. If you used 1/2 cup each of boiled and raw linseed oil, use up to about four cups of whiting, depending on the desired consistency.
- Mix in about 1 tbsp of stove or lamp black, depending on how dark you want the cement.
- Mix everything together until there are no streaks. Use your gloved hands when it gets difficult to mix with the spoon.
Comments:
The use of boiled linseed oil suggests an intention to increase the drying time of this formulation. However the lack of a volatile drying agent (such as turpentine or white spirits) means this will not have a particularly fast drying time which may cause disappointment. This recipe is substantially the same formulation as Recipe 2 and it is likely that one is a re-statement of the other.
Recipe 2
This is another simplified traditional style formulation. It is the essence of what I read here.
Ingredients:
- 1/2 cup boiled linseed oil
- 1/2 cup raw linseed oil
- 4 cups whiting
- Stove black or black colouring for cement
- A container for mixing the [not] putty and rubber gloves or nitrile gloves.
Method:
Keep adding whiting to the linseed oil mixture, a cup or so at a time to begin with then smaller increments towards the end. Mix using a spoon with gloved hands, much like making bread dough, until lumps are gone and the mixture is smooth.
If you want your black cement, add the black colouring at the end. If you do not use black, the cement will be a light beige colour like traditional glazing putty.
Comments:
The use of boiled linseed oil suggests an intention to increase the drying time of this formulation. However the lack of a volatile drying agent (such as turpentine or white spirits) means this will not have a particularly fast drying time which may cause disappointment. This recipe is substantially the same formulation as Recipe 1 and it is likely that one is a re-statement of the other.
Recipe 3
This is a faster drying traditional formulation. It comes from a blog.
Ingredients:
- 7 parts by volume whiting
- 1 part by volume boiled linseed oil
- 1 part by volume white spirits (turpentine or substitute or mineral spirits)
- 1-2 Tablespoons lamp black or other colorant
Method:
Add the whiting (reserving about one quarter) to the linseed oil and white spirit. Mix this well, by hand or with a domestic mixer capable of mixing bread dough. When these are mixed thoroughly, check the consistency is barely fluid. At this point, add the colorant then add more whiting as required to get the consistency you want.
The author of this recipe says 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, and that since making your own is cheap and quick to make, there is no saving in making a lot.
The author of the recipe also says that commercial cements have emulsifiers to keep the whiting from settling and so extend the life of the product. More about this later!
Comments:
Recipe 3 reveals a different (higher) proportion of whiting to drying agents than the previous recipes. The higher whiting proportion probably addresses the consequence of using a less viscous mixture of boiled linseed oil with volatile white spirit. The consequence of both boiled linseed oil and white spirit will be a faster setting than the first two recipes. This is a good starting point for your own experiments.
Recipe 4
This fast-setting recipe came from here but disappeared off the Web. It has been found again as the first of two recipes here. The author originally said “This putty sets up faster than commercial putty” which I take to mean it hardens more quickly than commercial leaded light cement.
Dry Ingredients:
- 4 parts whiting(chalk, calcium carbonate)
- 2 parts plaster of Paris
- 1 1/2 parts Portland cement
- 1/4 part lamp black
Wet ingredients:
- 1 part – boiled linseed oil
- 1 1/2 – 2 parts pure gum turpentine
Method:
The method implied is that dry and wet ingredients can be stored and then mixed to a desired consistency when required. This is a good idea, but the formulation isn’t.
Comments:
Recipe 4 is curious for several reasons. It is also bad for several reasons. Use it at your peril.
Specifying pure gum turpentine is needlessly prescriptive because it is expensive and will perform no better than ordinary turpentine or white spirits.
The use of Plaster of Paris and Portland Cement is not recommended, despite this formulation despite the claim it has a much faster setting time. The problem is that plaster of Paris and the Portland cement deteriorate in a damp atmosphere in the long term. A further problem with these materials is that they contribute to an excessively stiff cement which in turn raises the possibility of glass breakage. Another reason not to use Portland cement is that it strongly attaches itself to glass making future restoration activities more difficult. And in relation to your health and safety a problem is that Portland Cement becomes strongly caustic and can cause skin burns as well as severe eye or respiratory irritation. Health and safety problems also arise with the used of plaster of Paris.
Recipe 5
This is another fast setting recipe and is the second of two recipes found here.
Dry Ingredients:
- 1 part calcium carbonate (whiting/chalk)
- 1 part plaster of Paris
- 1/4 part lamp Black
Wet Ingredients:
- 1 part boiled linseed oil
- 1 part – white spirit (paint thinner)
Comments:
Recipe 5 is an improvement over recipe 4 but is still not recommended. One improvement is that less-expensive white spirits are now used. Another improvement is that Portland cement is no longer being used. The problem is with the Plaster of Paris.
The wet ingredients hint at a fast drying time and although the use of Plaster of Paris may be less hazardous than Portland cement, it is still not appropriate for reasons outlines in the previous recipe. Use this recipe at your peril.
Recipe 6
This recipe came from a book called Decorative Glass by Nance Fyson (page 164) and is interesting because it starts with traditional glazing putty and turns it into leaded light cement.
Ingredients:
75% linseed oil putty – traditional glazing putty
10% white spirit and 5% linseed oil mixed
10% lampblack
Whiting as required
Method:
Place in a bucket. The white spirit and linseed oil mixture is added to the lampblack then putty is added, a little at a time. To adjust consistency, more white spirit to thin or whiting to thicken. To darken add more lamp black.
Comments:
Recipe 6 is a practical and convenient suggestion in that the formulation starts with inexpensive traditional glazing putty and adds other materials to formulate a sound and sensible leaded light cement.
The use of white spirit is cost-effective and the extra linseed oil could be boiled linseed oil would improve the drying time. The addition of the white spirit and linseed oil will soften the mixture to make it more suitable as a leaded light cement – and the exact amounts can be varied to achieve the desired consistency.
Though not stated in the original recipe, using black powder paint (or some other suitable oil-based blackening agent) in place of lampblack would make the recipe more easily accessible to any modern stained glass craft worker who doesn’t have a ready supply of lampblack or soot.
This recipe is recommended if you want a quick, convenient and economical method of making your own leaded light cement, particularly if you have some left over glazing putty and have easy access to the other ingredients.
Recipe 7
This recipe was found at Project Gutenberg in a book called Stained Glass Work – a text-book for students and workers in glass, by C. W. Whall. When you’ve downloaded it, search for “How to Cement a Leaded Light“.
Instructions:
Whitening, 2/3 to plaster of Paris 1/3; add a mixture of equal quantities of boiled linseed-oil and spirit of turpentine to make a paste about as thick as treacle. Add a little red lead to help to harden it, some patent dryer to cause it to dry, and lamp-black to colour.
Comments:
This recipe comes from the start of the 20th century so it’s old. It is interesting in that it does lend some credence to the traditional (though not necessarily common nor sensible) use of Plaster of Paris in leaded light cement. It resembles recipe 5 in that they both use Plaster of Paris.
The use of red lead is confirmed as a “metal drying agent” but is not a practical proposition for most stained glass workers given the toxic nature of lead compounds and widespread legislative restrictions on their use. However, the mention of lead compounds does link nicely to some modern leaded light cement formulations that mention lead as one of their ingredients.
The reference to a ‘patent drier’ is an unhelpfully vague reference perhaps to a catalytic drying agent (that accelerates the oxidative polymerisation of the linseed oil) rather than to a volatile drying agent such as turpentine (that simply evaporates quickly).
This is an old recipe that isn’t recommended, mainly because it uses Plaster of Paris and red lead.
Commercial Formulations
Seeking out manufacturer’s safety data sheets (MSDS) and looking closely of packaging labels sometimes provide us with useful hints about the formulation of their products. From such sources we get some hints about modern commercial leaded light cement products.
Here we learn that their “putty” is a dough-like compound of whiting and linseed oil which may contain small amounts of white lead, red lead, lampblack and/or litharge (which is lead(ii) oxide). Those lead compounds are probably intended as metal drying agents.
And here we discover a formulation that contains naptha, orange lead, mineral filler, linseed oil, a plasticiser and white spirit. The term “mineral filler” we can assume to mean “whiting”. Notice we find linseed oil and white spirit in this recipe. Notice also that there is orange lead as a metal drying agent but no mention of lampblack or carbon black.
As a simple confirmation, this web page suggests that commercial glazing putty is exactly what we’d expect – nothing more than a ” Homogeneous mixture of mineral filler and linseed oil” for which we again assume that “mineral filler” actually means “whiting”.
Arbo leaded light cement used to be described here but the web page has now disappeared. It used to say that their leaded light cement contained a vegetable oil, mineral fillers, white spirit and metal driers. For the term “vegetable oil” we can assume “linseed oil”, for “mineral fillers” we can assume “whiting” and for “metal driers” we can assume a special metal drying agent. This metal drying agent will be chosen to catalyse (ie accelerate) the drying process by a process of oxidative polymerisation of the linseed oil (which makes the linseed oil thicken and harden) and may or may not be a lead compound of the kind seen in other formulations.
Comments here suggest there may be commercial formulations of “putty made for stained glass” that contain Portland cement and/or Plaster of Paris. The comments are, I assume, meant to offer us advice relating to commercial formulations of “leaded light cement” rather than “leaded light putty”:
If you are using a commercial putty made for stained glass, look for one that does not contain cement or plaster of paris. A good indicator of what not to buy is if the product contains the word cement in the name or description.
Although the author of that web page is correct to suggest that [Portland] cement and Plaster of Paris are a bad idea, the remaining comments are of dubious value because of the confused use of “putty” and “cement”. Think about it – the recommendation is not to purchase a properly formulated leaded light cement because there’s the word cement in the name – this is patent nonsense.
Although the web page here is now long gone, the forum thread had a similar comment about commercial “putty” (as usual, actually meaning leaded light cement):
Everyone has their personal favourite formula or brand. If you elect to buy premade putty [meaning leaded light cement], take care to read the list of ingredients. Reject ANYTHING that contains plaster or cement. Both are NOT waterproof but will absorb moisture and generate mold and mildew.
This time the author correctly limits product rejection to just those with ingredients that contain the words plaster or cement rather than in the name of the product. The comment about the water absorbing nature of Plaster of Paris and of Portland Cement is both useful and correct and will be described in more detail later. Again the key message is not to use leaded light cement with Plaster of Paris or Portland Cement as ingredients.
The web page here from which Recipe 3 was taken, says that “commercial cements have emulsifiers to keep the whiting from settling and so extend the life of the product”. I have found no evidence from any MSDS documents to support this claim. I also think it is unlikely to be correct because two immiscible liquids are required to form an emulsion and although we have oil-based liquids we don’t have water-based liquids in leaded light cement.
Another commercial stained glass cement formulation is a two-part American product called Miracle Mudd. You will find related MSDS documents here for the dry component and here for the liquid component.
Information gathered from the MSDS documents is typically sparse. This is not unusual because they don’t want you to know what you’re paying for, especially not when it’s “nothing special”.
The MSDS document for the dry part tells us it’s a grey powder that’s not reactive and not appear to be particularly hazardous to our health. If there were Plaster of Paris or Portland Cement involved I would expect the health hazard data and control measures to be greater than described. I would therefore speculate that it’s a mixture of whiting and some kind of colourant powder such as poster paint, brick colourant or perhaps even lampblack. So, nothing special here so far as I can tell.
The MSDS document for the liquid part tells us that it is an “amber to dark brown oily liquid with a pine aroma” with a boiling point of 307°F and flash point of 105°F. Also, an advertising claim is that it isn’t so smelly as competing products. Pulling these facts together is suggestive of the liquid part having a formulation that involves turpentine and linseed oil or derivatives of them. So, nothing special here so far as I can tell.
In conclusion, my guess is that Miracle Mudd is a traditional leaded light cement that you happen to purchase as separate dry and liquid parts and mix yourself. In this respect it is reminiscent of recipes 4 and 5 but probably without the nasty Plaster of Paris or Portland Cement. So, the miracle of Miracle Mudd would appear to be that it comes in two parts which doesn’t really seem to be such a great miracle.
Before I completely bore you to tears I think it’s time to stop looking at commercial formulations and pull together some general comments about the formulation of leaded light cement.
General Formulation Comments
A potentially good idea hinted-at by the authors of recipes 3 and 4, as for Miracle Mudd, is that dry and wet ingredients can be prepared and stored separately. This allows you to only make-up the amount of leaded light cement that you intend to use which in turn should reduce the amount of waste.
If you are making your own leaded light cement try to make only what will be needed for the current project. If you purchase ready-made leaded light cement try to buy quantities that are not excessively large. The problem with leaded light cement is that the whiting slowly separates from the oily liquid portion, resulting in a solid mass hiding under an oily surface. This separation is not permanent and can be reversed– loosening the solid portion, breaking it down and re-mixing it back into the oily portion with something like a palette knife or wallpaper scraper works but it takes a lot of time and patience.
Since making your own leaded light cement is quick and cheap, there is no saving in making large amounts in a single batch. If necessary, as hinted-at by some recipes, make-up and store dry ingredients and wet ingredients in distinct containers. Or consider the recipe from the book which adds ingredients to ordinary glazing putty.
One commentator suggests that a batch of leaded light cement has a shelf life of just one day but does not say why this should be so or the conditions that cause this to be the case. It is however plausible that formulations containing Plaster of Paris or Portland Cement may chemically ‘go off’ in consequence of the effects of atmospheric water moisture. If you’re sensible and are not using Plaster of Paris or Portland Cement then this should never be a problem.
Whether you are using a commercial glazing putty or a commercial leaded light cement, look for one that does not contain Portland Cement or Plaster of Paris. Although none of the MSDS documents I examined suggested that commercial formulations contain them, it does not mean that none of them do.
If your cement mixture is too stiff add just a little more white spirit to get it to your desired consistency. Conversely, if the mixture is too soft or runny add more whiting to stiffen the mixture.
If you are making a big batch of leaded light cement the mixing task can get physically difficult. In such circumstances using a bucket and an electric drill-powered paint stirrer becomes preferable to using a kitchen blender, kitchen mixer or a spoon.
And that’s all the advice I can give other than to suggest you experiment, do your own research and come to your own conclusions. I am not the fount of all knowledge!
For the remainder of this blog I’ll change the focus to an examination of the various component materials. Expect some repetition but I hope there’s not too much for you to bear.
What Is Whiting?
Whiting forms the bulk of the leaded light cement used to waterproof leaded lights in much the same way as whiting forms the bulk of glazing putty used for a traditionally glazed window.
Whiting is also used as a desiccant or drying agent to remove excess oils from leaded light cement during the “cleaning-up” process, thus promoting a faster setting time. Whiting serves as a mild abrasive to help remove excess cement and to polish the glass and leading as part of the final cleaning process.
Whiting is an archaic name for finely powdered white chalk. It has the chemical name calcium carbonate and the chemical formula CaCO3. However, not all “chalk” you’ll find being sold is calcium carbonate. So, be careful when purchasing “chalk” because it can also refer to other compounds including magnesium silicate and calcium sulphate. Although traditionally composed of natural chalk, modern blackboard chalk is generally made from the mineral gypsum (calcium sulphate).
Chalk is used to marking lines on playing fields and although some of these products are still made with calcium carbonate, it is titanium dioxide that is often used nowadays. Sports such as gymnastics, rock-climbing, weight-lifting and tug-of-war also make use of chalk though nowadays it is usually magnesium carbonate.
Whiting should not contain any calcium oxide (quick lime) or calcium hydroxide (slaked lime or hydrated lime). Calcium carbonate is a neutral substance that can be handled easily and safely but calcium oxide and calcium hydroxide are caustic alkaline substances that can be hazardous.
The building trade also use line marking chalk which may (or may not) be calcium carbonate. Painting and DIY outlets may have calcium carbonate (in stock or to order) where it will probably be sold as whiting or chalk dust. Pottery supplies stores may sell calcium carbonate where it will probably be called calcium carbonate or chalk dust.
Agricultural or feedstuffs merchant and garden centres may have whiting but be sure it is calcium carbonate (chalk) and not calcium oxide (quick lime) or calcium hydroxide (slaked lime) or some non-specific formulation such as “field lime” that may contain calcium oxide or calcium hydroxide. Also be wary of “field lime” because it can be very gritty and scratch your glass.
Lime plaster is a mixture of calcium hydroxide, sand and other fillers. The calcium hydroxide reacts exothermically with water and ultimately forms calcium carbonate. Whitewash is the same chemistry. As both involve calcium hydroxide they should be avoided in stained glass crafts. Despite this, some workers use lime, Plaster of Paris and related materials in their stained glass cement to achieve a faster setting time even though such formulations are more hazardous and less suitable than those that use whiting.
So, perhaps the best place to buy whiting for stained glass crafts is from a reputable stained glass store where the purpose for its use is well understood.
What about Plaster of Paris?
Plaster of Paris is also known as Gypsum Plaster because is produced by heating a type of rock called Gypsum. Plaster of Paris is more properly known as calcium sulphate semi-hydrate, with the chemical formula CaSO4·0.5H2O which tells you it contains just a little water. Adding water to Plaster of Paris turns it back to Gypsum, which has the chemical formula CaSO4·2H2O which means it now contains more water. So, in essence, Plaster of Paris relies on water to turn it back to “rock hard” gypsum.
Therefore the first and obvious question to ask is how it could be possible that Plaster of Paris might be a suitable component for leaded light cement when all the other ingredients (linseed oils and the drying agents such as white spirits) do not contain water. The chemistry of Plaster of Paris tells us that water that is needed to make it harden but the only source of water we have available must be rain and damp air. Something is not right here – we can not expect Plaster of Paris to contribute to waterproofing a leaded light panel if it needs water to make it harden. The only way the Plaster of Paris can “work” in leaded light cement is if it “compromises” the ability of the cement to repel water and moisture.
Another less obvious problem with Plaster of Paris is that it hardens to a hard and very inflexible state which is not desirable in a well-formulated leaded light cement. In well-formulated leaded light cement the resulting matrix of whiting in polymerised linseed oil is waterproof and strong but not so rigid as solidified Plaster of Paris. So, leaded light cement containing Plaster of Paris is more likely to crack the glass in leaded lights because it is too rigid. This is reflected in comments here where it says that “recipes that use plaster of Paris or [Portland] cement are not recommended because the product is too hard and stiff when dry, providing no cushioning for the glass inside the lead frame. A slight rubberiness is desirable to provide support without the stiff restraint of the cement, which can cause the glass to break.”
In summary, Plaster of Paris should never be a component in your leaded light cement.
What about Portland Cement?
Portland cement is a “hydraulic cement” meaning it hardens when exposed to water, becoming water-resistant (rather than waterproof). It is what you use to make concrete.
Quicklime, or calcium oxide, CaO, is a major component of Portland Cement and as such its nasty stuff. This makes Portland Cement highly alkaline which means it is strongly caustic and will cause skin burns if not promptly washed off with water. Similarly, dry cement powder in contact with mucous membrane can cause severe eye or respiratory irritation.
Everything that you just read about the use of Plaster of Paris also applies to Portland Cement.
In summary, Portland cement should not be a component in your leaded light cement.
What is Linseed Oil?
Linseed oil is a major component of both glazing putty and leaded light cement and is also known as flax seed oil. You can read lots about it at Wikipedia.
Linseed oil is what is known as a “drying oil”, which means it polymerises over a period of time (days or weeks) to form a solid mass. This polymerisation causes glazing putty and leaded light cement to harden to a rigid (rather than brittle) state over the course of several days. As a triglyceride, the water-repellent (hydrophobic) nature of linseed oil is also an advantage for its water-repellent weatherproofing properties. Read more about “drying oils” here if you have the enthusiasm because it’s the key to how leaded light cement achieves what we’re after – water-repellent strength without excessive rigidity.
Linseed oil is available either as raw linseed oil or boiled linseed oil. Boiling linseed oil causes it to partially polymerise and oxidise, making it thicker which in turn means it has a shorter drying time. Most products labelled as “boiled linseed oil” are now a combination of raw linseed oil, a volatile petroleum-based solvent (which evaporates quickly) and metallic dryers (which are chemical catalysts that accelerate drying).
As noted at the long-gone Stained Glass Town Square forum, raw and boiled Linseed oils are available (in-stock or to order) from a painting and decorating or DIY outlet. Art suppliers are also a source but are usually more expensive. Some agricultural or animal feed merchants might sell raw linseed oil as a feed supplement for horses.
What About Volatile Drying Agents?
Leaded light cement formulations often include turpentine, turpentine substitute or white spirits. There is little (if any) practical difference between these different products when used as a volatile drying agent in leaded light cement, the significant differences being how they smell and how much they cost.
Turpentine is a natural product derived from pine trees. As a natural product it tends to be expensive, particularly when highly purified.
Turpentine substitute (mineral turpentine) is an inexpensive petroleum-based replacement for turpentine. It consists of a complex mixture of highly refined hydrocarbon distillates mainly in the C9-C16 range, typically containing at least 60% white spirit and 20-40% naptha (petroleum). Low odour turpentine is close to 100% white spirit.
White spirits are the inexpensive petroleum-based replacement for turpentine, consisting of a complex mixture of highly refined hydrocarbon distillates. So, in addition to being cheaper, white spirits are less flammable and less toxic than turpentine and turpentine substitute.
Therefore, it seems, any of turpentine, turpentine substitute or white spirit will suffice but white spirit causes less smell and is the cheapest.
Paint thinners are a solvent commonly held to mean white spirits, turpentine or turpentine substitute. It can also be taken to include other solvents such as acetone, xylene and toluene. So, be aware that when you want to clean your glazing brushes paint thinners may not always mean white spirits.
What about Colourants?
Traditional glazing putty is a pale brown colour but leaded light cement tends to be shades of grey tending towards black. The colour was traditionally introduced by the use of lampblack which is nothing more than soot, which in turn is just powdered carbon.
Comments from the long-gone Stained Glass Town Square forum suggest that leaded light cement can be colored with stove blackening products (such as tubes of Zebo or Zebrite), concrete colouring (powder), powdered poster paint, oil paint and the traditional lamp black (powder).
This forum thread also warned against the use of any water-based pigment because they cause problems with linseed oil putty (because water doesn’t mix well with oils!). This also means that latex and acrylic colours are not acceptable.
What about Other Additives?
Lead and lead oxides were used in traditional formulations for leaded light cement but due to the toxicity of lead compounds their use is less common in modern formulations.
Why lead and lead oxides were used is not clear but one rational possibility for using them might be that they are toxic to all kinds of life including algae, moss and lichens that might attempt to establish themselves on your leaded light. Of these lichens would perhaps be the most difficult to avoid as they are well-known for their ability to tolerate concentrations of heavy metals, such as lead, that kill other forms of plant life.
Another possible reason for using lead compounds in leaded light cement relates to their possible use as a drying agent. In this context they may be catalysing the oxidative polymerisation of the linseed oil – making the linseed oil harden more quickly.
Unless you have a particularly strong justification for using lead or lead oxides in your own formulations for leaded light cement I recommend that you stay clear of lead compounds in the interests of your health.
If you have the enthusiasm for a brain-stretching read, have a closer look at Oil Drying Agents at Wikipedia to understand more about how some additives help to accelerate the drying of oils.
Cheaper Oils and Solvents
Boiled linseed oil is cheaper and ‘cures’ faster than raw linseed oil. This is why some formulations use a mixture of both. Ultimately they both become the same polymerised material in your leaded light panel .
White spirits are cheaper and just as effective as turpentine substitute which in turn is cheaper and just as effective as pure natural turpentine. Ultimately they all serve the same purpose and will all evaporate out of your leaded light panel.
That’s All Folks!
Thank you for your patience reading this blog. I hope my chatterings have been useful. Do try some experiments of your own and let us all know how successful they were.
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