The topic of canvas filler formulas has arisen a number of times by me, as well as others and I decided to take a closer look at what's going on here. Using the published formulas at the wcha.org site, I'll try and ferret out the processes that are involved.

Firstly, let's look at the two most common components starting with silica which ranges from 1 to 1.56 kg per recipe. Of all the explanations for silica that can be found at Wikipedia (http://en.wikipedia.org/wiki/Silica), two could be of interest to us:

1. Silica, with alumina (silica-alumina), is a crucial ingredient in clay (http://en.wikipedia.org/wiki/Clay) and allows for the development of an interlocking crystal matrix after firing in earthenware, stoneware and porcelain ceramic processes.
2. Silica is a major ingredient of Portland cement.(http://en.wikipedia.org/wiki/Portland_cement)

So, what we are dealing with when we fill a canvas is really a process akin to “cementing” a boat, or creating a clay pot! The fact that we don't use a kiln is academic. Using a kiln is just supplying a catalyst, just like using a two-part epoxy, and its hardener. In the wooden canoe case, we use other components to generate the needed chemical reactions instead of the heat of a kiln.

The second component is linseed oil, and again, Wikipedia (http://en.wikipedia.org/wiki/Linseed_oil) provides these two nuggets:

1. Boiled linseed oil was used as a paint binder or as a wood finish on its own. Heating the oil makes it polymerize or oxidise more readily, effectively shortening the drying time.
2. Linseed oil is a yellowish drying oil derived from the dried ripe seeds of the flax plant.

So, since the silica is a dry, powdery ingredient the two characteristics of linseed oil we're really interested in is maybe that it is a “drying oil” and a “binder”.

A drying oil (http://en.wikipedia.org/wiki/Drying_oil) is defined as:

1. A drying oil is an oil which hardens to a tough, solid film after a period of exposure to air. The term "drying" is actually somewhat of a misnomer, since the oil does not harden through the evaporation of water, but through a chemical reaction in which oxygen is absorbed from the environment (autoxidation). Drying oils are a key component of oil paint and many varnishes. Some commonly used drying oils include linseed oil, tung oil, poppy seed oil and walnut oil.

To go a bit further into it:

“The drying of oils is the result of a reaction, called autoxidation, which is chemically equivalent to slow, flameless combustion. In this process, oxygen attacks the hydrocarbon chain, touching off a series of addition reactions. As a result, the oil polymerizes, forming long, chain-like molecules. Following the autoxidation stage, the oil polymers cross-link: bonds form between neighboring molecules, resulting in a vast polymer network.”

Now, the term “polymer” may scare some off because we (I anyway!) immediately think of modern plastics, but in this case all it refers to is the creating of bonds between molecules and as these bonds keep forming its what paint does when it “dries”; as the bonds keep forming the links between the molecules keep getting stronger and stronger until (to us) it appears to have solidified.

While we're here and I mentioned “plastics”, here two more definitions for you:

1. In solid mechanics, a material behaves elastically if it changes shape due to an applied load, and that when the load is removed, recovers its original shape.
2. ...plasticity is a property of a material to undergo a non-reversible change of shape in response to an applied force.

I didn't know that.

Now, before we sum up all this, let's look again at silica which is made up of:

1. Quartz is one of the most common minerals in the Earth's continental crust. It has a hexagonal crystal structure made of trigonal crystallized silica (silicon dioxide, SiO2), with a hardness of 7 on the Mohs scale. Density is 2.65 g/cm³. The typical shape is a six-sided prism that ends in six-sided pyramids, although these are often twinned, distorted, or so massive that only part of the shape is apparent from a mined specimen.(http://en.wikipedia.org/wiki/Quartz)

So, if we mix these two components we would essentially be mixing clay with a liquid oil which reacts with oxygen in the air and that reaction is equivalent to a slow, flameless combustion (kiln).

Now, a third common ingredient in our recipes is Japan Drier (see refs on it) and one of the references states their product is 97% Petroleum Naptha, and 3% Soluble Cobalt Compound. From what I can determine the cobalt is used as a heavy metal catalyst for this polymerization reaction which goes on in the drying oil. Sort of like “stoking the fire” of this slow, flameless combustion. I have to assume the naptha is merely a solvent, i.e., a carrier of the cobalt because being so volatile (“...evaporates at a relatively low temperature...”), it won't be around too long!

Also of note here is that if you read the Wikipedia on linseed oil, you would have noted that:

1. Today most products labeled as "boiled linseed oil" are a combination of raw linseed oil, petroleum-based solvent and metallic dryers. The use of metallic dryers makes boiled linseed oil inedible. There are some products available that contain only heat-treated linseed oil. These are usually labeled as "polymerized" oils though some may still be labeled as boiled.

So the use of linseed boiled oil already has some heavy metal catalysts in it. This may or may not affect the recipes. (Some recipes have twice as much Japan Drier than another so this accelerated drying may make the filler “dry” too fast and “...will wrinkle the paint film if too much is used.” Indeed, Rollin Thurlow pointed this out on the wcha site, see below.)

It seems to me at this point we already have everything we need for a complete canvas filler but the recipes add more. The next is either turpentine, or mineral spirits. These are defined as:

1. Mineral Spirits is a petroleum distilate (sic) commonly used as a paint thinner and mild solvent. (http://en.wikipedia.org/wiki/Mineral_spirits), and
2. Turpentine is a fluid obtained by the complex distillation of resin obtained from trees, mainly various species of pine (Pinus). The two primary uses of turpentine in industry are as a solvent and as a source of materials for organic synthesis. (http://en.wikipedia.org/wiki/Turpentine)

So what we're adding to the cake mix at this point is a solvent: either one made from petroleum, or from trees. So what a solvent does is get the other ingredients into solution (http://en.wikipedia.org/wiki/Solution), because (I guess) the silica and oil don't mix or blend with each other real well.

It's at this point that it seems we should have a good filler but two of the quoted recipes add lead (http://en.wikipedia.org/wiki/Lead) to the filler. I can't see any listing on the Wiki page of a use for lead in this concoction we're making here.

But, I found it interesting that the amount of lead in the three recipes is odd: if we take the recipe with none as 0 and the one with the largest amount as 1.0, the middle falls on the 0.77 amount.

If we take the silica in the recipe and chart it similarly we have 1.0, 0.8, and 0.64. As the amount of lead goes up, the amount of silica goes down, pretty well on a predictable variation.

Starting at the fully “leaded” version, if you use one more pound of silica, you can reduce two pounds of lead, and if you use another pound and a quarter of silica, you can reduce the lead a further two pounds to zero.

So it seems to me adding 2.25 pounds of silica and dropping 4 pounds of lead sounds like a bargain. The leaded version also requires double the amount of turpentine so I imagine 1.5 quarts less of that would save weight as well (although if it all vaporizes there would be no gain).

Now, even though at the Wiki I could find no use for lead in our formula, we are well aware that it is useful as a toxic agent to make the canvas an unliveable environment for rots, molds, and such.

The other use I see is that the drying oils needed a heavy metal catalyst to accelerate the polymerization process and the most familiar heavy metal to most people who probably don't know it, is...lead, (Okay, maybe it's “iron”.).

(I later found whilst searching on “white lead”:

1. Lead was added to paint as pigment and to speed drying, increase durability, retain a fresh appearance and resist moisture which caused corrosion. (http://en.wikipedia.org/wiki/Lead_paint))

We can see this from the amounts of Japan Drier used: 7 ounces for the fully leaded version and more than double that for the half-leaded version. So we can assume there was enough heavy metals around using 4 pounds of lead that the requirement for additional cobalt in the Japan drier was significantly reduced.

But getting back to the recipes, the one with no lead also has two ingredients not found in the other two: enamel paint, and spar varnish.

A varnish (http://en.wikipedia.org/wiki/Varnish) is:

1. “... usually a combination of a drying oil, a resin and a thinner or solvent.”

Enamel paint (http://en.wikipedia.org/wiki/Enamel_paint) is described as:

1. An enamel paint is a paint that dries to an especially hard, usually glossy, finish...Some enamel paints have been made by adding mouse (?) varnish to oil-based paint...
2. The term sometimes refers to oil-modified polyesters that were introduced in the early 1930s. The oil is required to stop or enhance the crosslinking of the paint in order to achieve sufficient flexibility of the paint film.

So we can say that the varnish or enamel paint add MAYBE some extra drying oils for polymerization and since both have it, some amount of Japan drier or other metallic drier.

So where the hell, you may ask, am I going with all this?

Well, there certainly are a number of experienced people with literally hundreds of terrific solutions, hints, suggestions and answers, but a definitive canvas fill reasoning was not amongst them.

And the individual ingredients are pretty well researched and known, but not in relation to using them as a canvas filler. For example, linseed oil as a base for fine art oil paint is pretty well known, but what we want is a drying oil and just because “boiled linseed oil” is a drying oil and it's been part of the recipes, do we really know why it was chosen versus another oil? I'll bet these decisions were not made by figuring out what's best but by other factors like, "What do we have available?" Or "What's cheapest and does a reasonable job?"

So, let's face it. From everything we've found out, it still seems like the canvas filler formula is pretty much a hit-and-miss, trial-and-error calculation, but we should be able to come up with a better mix if we think about this a bit...

I think one drawback to the current filler is the “4 week curing period”. Another is the massive amounts of solvents used. For example, “Rushton's Filler” lists 1½ quarts of solvent and 1 pint of Japan Drier. Now, from what we found out above Japan Drier (in one case anyway) is 97% solvent and 3% cobalt. So the recipe reads:

1. Combine 48 ounces of solvent with 15.52 ounces of solvent, and 0.48 ounces of soluble cobalt.

Now, as pointed out above, the only function of a solvent was to get all our other ingredients to blend evenly and (probably) into a smooth workable mix. Solvents will only be in our mixture until they evaporate, so we should treat them as temporary and use only as much as we need and the instruction for a solvent would be similar to “add salt to taste”, i.e., forget stating a specific amount and go by what's needed to create a workable “paint”. It's also be more environmentally-friendly because if all we need is less than a half-ounce of soluable cobalt, what's the other cup of solvent for other than to carry it into the mixture and evaporate?

If we forget about the amount of solvent for the moment, should we prefer say, turpentine over mineral spirits? I'm un-decided at this right now but below I make a preference.

The oil we need has to be the best at polymerizing, creating all those cross-linked bonds to end up with a well-structured tightly-bonded coating that hold our silica, or other material, filling the canvas.

Our drier, our heavy metal catalyst, should be a “through drier” as opposed to a “top drier”, because top driers cause a polymerization on the surface of our application which hinders atmospheric oxygen from reacting quickly, or satisfactorily, with the deeper molecules until much later. This could result in an extended cure time. Lead, for example:

“Lead functions as a powerful drier by promotion polymerization of drying oils, causing the film to dry in its entire thickness.; in other words the drying of the surface and inside the film is catalyzed uniformly. Lead is, therefore called a "through" drier like cobalt is known as the top drier.” (http://www.durachemicals.com/driers1.htm)

So why would the recipes prefer Japan drier with its top drier versus whatever else is available with a through drier?

If we use a top drier which creates a film at the mixture/atmosphere interface, should we be suprised that evaporating solvents may cause bubbles or blisters?

If we jump back to oils for the moment:

“The nomenclature in the area is arcane and confusing. Raw linseed oil has actually been processed to remove undesirable materials. "Boiled" linseed oil has cobalt and manganese driers added to make the polymerization process faster. Its films can set up in hours, rather than days. Older boiled linseed oils had lead salts as the accelerator, but these have been abandoned because of toxicity. Unfortunately, cobalt driers can lead to frosting, since "top drying" of the film is promoted.” (http://www.recorderhomepage.net/wood.html)

Now this is from a description on oiling wooden musical instruments but frosting is of no concern to us, is it?

So from what I see our recipe should be more logical. Compare:

Ingredient	Which is Really	What We Need
Boiled linseed oil	Oil Petrochemical solvent Top drier	Oil
Japan drier	Petrochemical solvent Top drier	Through drier

(Remember solvents only help the oil and drier mix properly, plus admittedly stretching the mix and making it more penetrating by thinning it out.)

“no lead” recipe	Which is Really	What We Need
43 ounces boiled linseed oil	Oil Petrochemical solvent Top drier	Oil
21 ounces mineral spirits	Petrochemical solvent	Solvent
34 ounces enamel paint	Oil Pigment Top drier	Through drier
2 ounces Japan drier	Petrochemical solvent Top drier
2 ounces spar varnish	Oil Petrochemical solvent Top drier Resin
6 1/4 pounds 300 grit silica	...silicon dioxide... ...quartz... ...a crucial ingredient in clay... ...a major ingredient of Portland cement...	...one of those!

At this point, I can see no reason why one should prefer mineral spirits (petrochemical) over turpentine (resin distillate) but because our varnish is already a resin distillate and enamel paint probably uses turpentine in its recipe (or so I've read elsewhere.), seems to me might as well stick with the turps.

Finally, back to oils, the reference 5 below explains the following:

1. Drying oils, including linseed and tung, can be defined as liquid vegetable oils that, when applied in thin layers to a non-absorbent substrate, will dry in the air to form a solid film. This drying is a result of polymerisation by the action of atmospheric oxygen, i.e. autoxidation. The films are usually hard, don't melt and are insoluble in organic solvents (this varies with the drying oil)...Drying oils are usually sub- divided into three main groups - non-conjugated, conjugated and other oils.
2. Non-conjugated oils, such as linseed...are fatty oils that contain polyunsaturated fatty acids, whose double bonds are separated by at least two single bonds (i.e. isolated double bonds make up the non-conjugated oils).
3. Conjugated oils, such as tung...are polyunsaturated fatty acids whose double bonds are partly or fully conjugated (that is, alternate single and double bonds in the carbon chain are the fatty acids).
4. The high amount of linolenic acid in non-conjugated oils, like linseed oil...affects its drying characteristics. High concentrations of linolenic acid can result in rapid drying, yellowing and brittleness.
5. (Non-conjugated)...(o)ils with low or no linolenic acid, like soybean and safflower oil, obtain their drying characteristics from high levels of triglycerides, which contain linoleic acid. The drying of these oils produces flexible films with little yellowing.
6. Conjugated oils like tung oil are much more reactive than non-conjugated varieties. Conjugated double bonds favour polymerisation and oxidation and dry faster than non-conjugated oils, and the film offers a high resistance to yellowing plus greater resistance to water and alkalis.
7. The main drying component in tung oil is eleostearic acid, a conjugated octadecatrienoic acid. The oleic acid in the fatty oils and unsaturated fatty acids plays a small part in the drying process too. The saturated fatty acids present, however, act only as plasticizers.

Now, that said "Conjugated oils...are much more reactive...(and)...Conjugated double bonds favour polymerisation and oxidation and dry faster...and...offers...greater resistance to water and alkalis." This seems to be exactly what we're looking for in a polymer oil.

And that ain't linseed oil.

If we add the liquid quantities in the above recipe we get:

43 ounces + 21 ounces + 34 ounces + 2 ounces + 2 ounces = 102 ounces

If we take the solvent out of that (what we can anyway), it's:

43 ounces + 34 ounces + 2 ounces = 79 ounces

That seems to me to be the maximum amount of oil we need because all three of those ingredients already contain some solvents already blended in them.

Now, in the interests of economy, if that's the maximum, I'm thinking maybe we could reduce that even further to say, 64 ounces which is 1/2-gallon, so if we play our cards right, we could do two canoes with a gallon of oil. And from what I can see, a conjugated oil that's been polymerized ("Polymerization is essentially a cooking process that enhances the molecular structure of the oil and further improves the natural cross-bonding reaction of raw...Oil.") is the best choice with Tung Oil at the top of the list.

For our solids, I've not looked into other mixtures and will stick with silica, but there are some other interesting compounds that someone might want to look at. As mentioned above, Portland Cement is worth investigating because "Calcium silicates or silicate constituents make up over 70 % by mass of silicate-based cements." These silicas are what we were looking for to mix with oil, so wouldn't water be easier? There's enough references to cement boats out there to choke a horse...(the other analogy was just too blatant to use!)

So we leave the silica at whatever volume 6 1/4 pounds is for now.

To get this goo to blend and flow, I'd go with turpentine "to taste".

The final additive is a through-drier if we can find one. The lead option is awful tempting, and I'm not sure that there could be all that much toxicity. Someone can maybe get in touch with a polymer-guy and ask him if the lead can really get out of a polymer other than being scraped off. I guess scrapping the canoe on rocks or the bottom would add some lead to the enviroment but how much and to what result, I've no idea.

From the recipes, if we add lead we can reduce the silica or so it seemed from above, besides if the Japan drier was only contributing .48 ounces of cobalt to some already probably small amount of drier in the oil, varnish, and enamel, really how much of this optimum through-drier do we really need to add? I can't imagine that a single pound of lead added to the mixture would be a great environmental concern but that'd be a great addition to make the filler dry throughout and quickly.

As the amount of lead went down in our original recipes, the Japan drier went up and so did the silica. I think replacing a through drier by doubling the top-drier was an attempt to make things solidify quicker, but the wrong way to go.

Obviously, the cost may go up with a recipe of tung oil, silica, turpentine, and lead. I found the best advertisement for Tung oil at Sutherlands Ltd. http://www.sutherlandwelles.com/tungoil.htm which would lead a suggestable reader to their product, which is also sold under a Lee Valley Tools Ltd. http://www.leevalley.com/ brand.

Sutherland further claims:

"Sutherland Welles Ltd.® formulates the entire line with the "cleanest" solvents and the lowest toxic driers available. We are committed to "clean and green" and finisher-friendly products!"

I dunno what they'd say about us adding lead to their product! ( I emailed them asking what type of solvent, i.e. petro/resin, and drier, i.e. top/through, they used in their oil.)

Anyway, take this for what it's worth and it'd be interesting to mix up a batch and see what happens, eh?

I've no vested interest in Sutherlands, or Lee Valley, by the way....;)

References

Japan drier

1. http://www.opusframing.com/library/pdf/choosing_oil_paint_mediums.pdf

2. www.utrechtart.com/msds/docs/JD.pdf

Oils

1. http://www.woodenflute.com/maintaining

2. http://howthingswork.virginia.edu/page1.php?QNum=748

3. http://www.sutherlandwelles.com/tungoil.htm

4. http://www.sydneywoodturners.com.au/site/articles/finishing/oils.html

5. also http://www.arbortech.com.au/articles/013.html

Natural tung oil is slow curing, and does not cure very hard, so it is best used when a thin matt varnish film is desired and the surface is not subject to heavy wear - it's great for pine wainscotting. Heat-treating ('polymerizing') tung oil solves all these problems - the final finish is almost as tough as urethane, dries almost as fast, bonds better and polishes better. There is only one drawback of polymerization - the oil is thickened so much that a thinner is required for application. Use something sold as a paint thinner for this...http://www.sankey.ws/pine.html

Glazing putty is traditionally made by mixing a base of whiting (finely ground chalk) with linseed oil in various proportions, however a number of synthetic alternatives exist. (http://en.wikipedia.org/wiki/Putty)

Linoleum, discovered in 1863 by Frederick Walton, is named after the flax plant (Linum) and oil (oleum). Oxidized linseed oil was mixed with ground cork and pigments, pressed onto a burlap or felt backing, and then baked. The tough, elastic, waterproof qualities of oxidized linseed oil gave linoleum its fine quality features. (http://waynesword.palomar.edu/chemid1.htm)

Industry needs

The paint and surface coatings industry already uses significant quantities of soya and linseed oil. In addition to the required functionality, industry needs raw materials to be available in sufficient quantity, with consistent quality and at a competitive price. For paints, varnishes and inks the oil needs to have a degree of unsaturation, that assists in the drying process, but not so unsaturated that the oils are unstable, easily oxidise. The ideal oil is one with a high level of linoleic and low levels of saturated fatty acids. Such oils are available from safflower and high linoleic linseed. The requirements for inclusion in a paint formulation include, consistent composition, good initial colour, high level of unsaturation without yellowing, available in bulk quantities, pumpable in non-heated lines, low tocophoral content, no significant price premium.

Specific opportunities exist for supply of polyols, such as from castor bean, for manufacture of both alkyd and polyurethane based paints and for conjugated fatty acids (such as from calendula), for formulations offering a high degree of reactivity.

Because of the need for continuity of supply and competitive price, industry has traditionally used commercially available oils, which are mainly produced for the food industry. (http://www.biomatnet.org/secure/Fair/S1075.htm)

Rollin Thurlow - “I would say that the japan drier, while it speeds the curing process it is limited in how effective that it can be. The canoe filler is put on much, much thicker than any paint manufacturer ever dreamed their paint would be used. The oils and thinners used to make the filler to be smoothed must evaporate out of the filler before being trapped by the finish paint. The top layer of the filler will harden very fast and this just slows down the ability of the oils and thinner deep in the filler to evaporate. Adding even more japan Drier may seem like a good idea but that will cause the filler harden and age prematurely and cause the filler to crack sooner than normal.

If you want a faster process, use something else!”