Perhaps the most fundamental technology lost from older painting has to do with how early painters refined and modified their oil. Later, as commercial tube paint made with raw oil became more and more prevalent, so did various resin or resin and oil mediums designed to alter the behavior of this paint. "The Artist's Assistant" by Leslie Carlyle makes it clear that in 19th Century England, the search for the older manner of painting resulted in a virtual minefield of complex materials. Whether older painters did or didn't use resin mediums such as amber varnish is a tangled web. However, the research carried out by the National Gallery and reported in their yearly Technical Bulletins consistently suggests that very little resin was ever used, that most of this was soft -- i.e. larch balsam -- and that the use was in "small amounts," possibly fused into the oil in some cases. This is logical: older painting is consistent in its technical conservatism and economy of means.

      However, having made and worked with amber, copal, and sandarac varnish for several years, and being somewhat enamored of this technology, this simplicity was at first hard to comprehend. How could it be done otherwise? Ultimately I decided it would make the most sense to learn an all-oil method in order to compare the techniques fairly.

       From what has come out of this experience, it seems fair to suggest that evaluating the potential of the oil based on the assumptions of mainstream modern painting practice is to begin and end in profound misunderstanding. Two fundamentally different systems exist.

       While the older system can never be known fully or with certainty, working with its principles via older recipes and procedures leaves no question that it offers a more functional and evolved way of utilizing the oil.

       This is because it begins with very high quality, hand-refined oil, and this oil leads to places systemically that modern oil cannot. The modern system features an emphasis on purchasing ready-made products, instead of understanding the behavior of basic materials at a functional level.

      As I learned more at this level, the limitations of the modern method became apparent in relation to the holism of older painting practice. For painters who are interested in the many ways the art and the craft inform one another in a practice, the information below will hopefully save you some time in exploring what the oil can do.

      Like many painters I first became concerned about the commercial version of linseed oil by observing it in action. The De Mayerne manuscript (1620 --1632) is full of recipes for fixing linseed oil so it won't yellow as much: this was obviously an issue with the oil that was still often preferred for its strength and availability. This, in addition to the color of the oil I saw oozing out of tubes of commercial paint, even very high quality commercial paint, always made me nervous about using commercial, "alkali-refined" linseed oil. I didn't doubt that linseed oil could be used in a non-yellowing manner: this was clear from older paintings and the analyzes by the National Gallery in their yearly Technical Bulletins. In fact, commercial linseed oil has gotten better over the years; several manufacturers now sell a cold-pressed oil which is ultra-refined, and in fact doesn't yellow. However, it has been so cleansed, so stripped, that it's also devoid of all character, has none of the snap or pop associated with OM linseed oil. Whether this is important or not depends on how relevant the older craft seems. The older craft begins with processing the oil.

       Painters are very fortunate now to have available several different choices in cold-pressed, unrefined, organic linseed oil. Oil of this quality was literally unavailable to 20th Century writers such as Taubes, Mayer, Laurie, or Wehlte, and it is important to view their comments on "linseed oil" from this perspective. What they are always talking about is the linseed oil of commerce from their period.

       My first experience refining linseed oil followed the guidelines in Eastlake and Merrifield, washing the oil using water, sand, and salt. It took six washings of one week each to remove all the fatty acid break. The washing was done using the cold-pressed, organic oil Allback oil from Sweden. Before and after photo above, lighter is after. For more on this process, which did result in a much quicker drying oil which yellowed to about the level of refined walnut oil, even less after aging a year in the light, see refining section below.

       This process should not to be confused with the various other, quicker processes -- breadcrumbs, lavender, alcohol, et al -- simply designed to clear an unprocessed oil, typically found now in a health food store. All of these methods have their origin in the older authors, from De Mayerne to Pacheco. And while my experience refining this type of oil has been very positive, I don't think these "quick and simple" methods are that safe long term. When the fatty acids remain in the oil they always present the danger of long term yellowing through the inevitable by-products of oxidation. Simply buying the highest quality oil is not enough, it still must be refined to remove the fatty acids.

      Oil was often used to make paint in a somewhat heat-bodied state -- see below and article on Van Dyck in National Gallery Bulletin 19. This is a logical, inexpensive way of increasing the gloss and elastic working characteristics of the paint in a thin layer while shortening the drying time. Between the almost watery thinness of raw oil and the too great viscosity of sun or stand oil lies a world of interesting possibilities which it seems older painters explored fully. While much of the original work I did with the materials concentrated on the potential of a hard resin varnish in the medium or paint, it was in fact the changes in paint rheology observed by using slightly heat-bodied oil to make paint which suggested that the medium was not as focal or as large a factor in the system when the paint itself has been effectively "resinated", giving it an elastic and prehensile working quality that is essentially the opposite of the false-gel "delicious butteriness" of commercial tube paint.

      Because of it's historical and continued commercial importance, a long history of scientific research into the behavior of linseed oil exists. Science has many long words, and is always quite confident of the validity of its results. For this reason, painters -- reeling from the half truths in the catalogs? -- will often look to the findings of the laboratory for more reliable information.

       In theory this is a great idea, but in practice this proves much more complex, especially with regard to the many scientific papers available now through the internet about linseed oil. This is because science is always highly subtle in its analysis, but not necessarily in its fundamental conception of the situation. The 17th Century scientific attitude -- the De Mayerne manuscript offers a legendary example -- was result or goal oriented. Materials were investigated with a specific practical end in mind, and De Mayerne provides a virtual encyclopedia of the 17th Century techniques of working painters for refining raw linseed oil. The current scientific attitude is grounded in the ability of the modern laboratory to look deeply into the way materials are constructed, and how their components interact. For this reason it is easy for modern science to have no practical purpose, or to miss the point.

       As a primary example, I have not been able to find a single scientific analysis of the behavior of linseed oil which begins with a cold-pressed, unrefined oil. This is of course logical, an unrefined oil must first be refined, which introduces a profound experimental variable. But all older painters began with cold-pressed, unrefined oil, and refined it in some way.

       If we grant that certain older paintings are in much better shape than they should be -- Rembrandt is the usual example because of his later manner, but there are many others as well -- the conclusions of science with regard to its abstract "linseed oil" are unfortunately of no application in this situation. Does a cold-pressed, unrefined oil behave the same way as the refined linseed oil of commerce? No, not at all. From the perspective of the painter looking for high quality, traditional materials, the reams of available analysis are therefore about the wrong thing. This is an example of the way in which specific oversimplifications make the "scientific" approach to painting materials problematic for working painters.

       The material to begin with is a cold-pressed, unrefined oil, the process to explore is how it can be minimally refined to dry with the least amount of yellowing. This is not that complex, it was part of the working life of painters for centuries. As painters drifted away from a working proximity to the oil, as commerce took over the oil and the paint in the 19th Century, the foundation was lost, and the craft in many cases fell apart. Working directly with the oil is the logical starting point for a painter interested in a renaissance of the personal craft designed to access the look, longevity, and especially the myriad sound technical options of older work.

      Above, chart of a commercial linseed oil refining process, from Scaldis in Belgium. In commercial oil, everything is removed. This can result in a non-yellowing oil, but, from the rheological perspective, this is overkill.

      Some of the oils generated by this research. Number one is the Spectrum Naturals Walnut Oil, washed, then heated to 100C for 48 hours. Not the greatest drier, but hardly yellows, I use this to make paint and as a putty medium ingredient. Two is the Allback oil, with an addition of commercial bleaching clay. While this took all the color from the oil, it still performs in every other way like raw oil. Three is the raw Allback oil, takes about 3 to 5 days to dry, yellows a little. Four is the Allback oil refined with water sand and salt. Process takes about five or six weeks, about one third of the oil is lost, resulting oil dries in a day or two with almost no yellowing, especially if it is aged in the light. Five is the Flora oil from British Columbia refined with litharge using an old method that also occurs in Field's Chromatography (1841). A quick drier with little yellowing. Six is the Allback oil, pre-heated to 100C for 24 hours, exposed to air in a thin layer in a lead tray for a week, then frozen overnight. The fatty acid break then falls out like snow, however, this can take a few months to fully clear. This oil has a nice balance of body and movement, loses all color on exposure to light and air.

       Allback markets a cold-pressed, organic linseed oil grown in Sweden, a country with exactly the right climate for producing high quality linseed oil. They also market paint for the exterior and interior of houses using this oil and pigment according to old Swedish recipes that they've researched. This is a high quality product. Although the nutritional oils are even better, they are also much more costly. If you would like information about this very high quality oil, the Allback website is here.

      A very high quality, reasonably priced oil is put out by Azure Farms in Oregon. This oil is very alkaline, smells like a field of flowers.

      A more readily available retail nutritional oil in America that has worked out well in use is the Flora brand from British Columbia. Expensive retail at 20 dollars a pint, available online at 25 dollars a quart. Very carefully processed, performs well in all the refining methods here.

      Another high quality nutritional oil is the Barlean's oil. These oils are being augmented with an increasing variety of cute ingredients. When shopping for this type of oil, be careful to get "plain", not high-lignin, lemon-flavored, strawberry shake, etc.

      Another high quality oil which is available cheaply in the USA is the Jarrow brand, organic and cold-pressed, packed under nitrogen in Washington State. Available for between 16 and 18 dollars a quart online.

       In 2006 I began working with the organic cold pressed linseed oil from Sweden marketed by Allback. I refined it several traditional ways in the hope of getting a fast drying oil that didn't yellow.

       Using this oil in the first place over the alkali-refined oil of commerce was a large plus, none of my samples ever turned the incredible orange color one sees in this type of commercial oil, but something does need to be done to it before it will dry readily. The first experiment was washing the oil with salt, sand, and water, a technique spoken of in many older books. This took six weeks to do, and a significant portion of the oil was lost, between a quarter and a third. This process removed a great deal of the water-soluble fatty acids, the "break," and is not to be confused with the various quicker processes designed to clear a cloudy, totally unprocessed oil. These processes leave a significant portion of the break remaining in the oil, i.e., almost all of it. Whether oil can then be made non-yellowing simply by traditional exposure to the sun is an idea experience with full-fatted sun oil casts doubt on.

       The refined oil does dry much faster than the unrefined oil, but it also yellowed at first. Much less than the alkali refined oil of commerce, a little more than walnut oil. After aging in a sunny window for a year, this oil was lighter and yellowed still less, the same amount as the Doak linseed oil and the Holbein linseed oil in tests -- very little. However, this oil was not treated with bleaching clay, whereas those oils are. I don't know whether bleaching clay or other aspects of the process are responsible for the thin, watery aspect of high quality commercial oil. But I like the slightly grippier, tighter working quality of the traditional oil better.

       So, again in line with the older writings, age may well be a factor as well as origin, method of extraction, and method of refining. I also noticed that this oil did not dry from the surface or form a skin, in spite of drying quickly, one and a half to two days.

      Is this process worth the effort? As with many things in painting, it depends. The linseed oil I refined is the best linseed oil I've used: it has a unique working character that made me understand, for the first time, the fact that linseed oil could be preferred over walnut oil by older painters. So, relatively long process though it is, it offers an avenue to an oil of this type. Are there other high quality linseed oils available that require no further processing? Yes. But they don't dry quickly. If you want to explore how the oil can be further modified, or you want to use the putty method and paint without solvents, refining it yourself using this method is a great place to begin.

      So far it seems that a combination of several of the traditionally known factors contribute to an oil which dries faster, harder, and with less yellowing. These factors are: a high quality cold pressed oil to begin with; refining using water,sand, and salt -- or some analogous method that remove a substantial portion of the break -- subsequent aging or preheating of the oil, and use of the oil in thin layers which will dry quickly. The one exception to the thin rule seems to be the chalk putty medium, which does not increase yellowing even when used thickly. More information on using the putty medium can be found here.

      The yellowing -- orange-ing? -- of linseed oil in several artist grade commercial paints. Significantly less yellowing in paint made by Robert Doak on the right for comparison.

      It's important to do yellowing tests of the oils, it takes about three or four months to find out the worst. I try to use the same ground I'll be working on, in this case a glue gesso. Yellowing tests are very dependent on relative humidity. Oils will yellow more with high humidity, less with low.

      This is based on the method outlined by Eastlake and referred to by Mrs. Merrifield in the 19th Century literature.

      Half a gallon of oil was placed in gallon jugs, along with 4 tablespoons of sea salt, 1 cup of fine sand, and 1 quart of hot, non-chlorinated water. The sand must be washed first to remove any dirt it may contain. Coarse pool sand will work better than standard hardware store sand. The jug is capped, then shaken vigorously, letting the pressure off by releasing the cap slightly. The jug is then placed on a low source of heat. I used several lab hotplates on their lowest setting, one for each jug. Other painters have reported using a food warming tray with good results. There is one older writer who puts the whole jug into a low oven, but I found this didn't work as well for removing the break as having the water warm, the oil cool.

      The jug is shaken every day, several times a day, with the pressure being let off each time. At the end of the first week a large portion of the break will have appeared between the oil and the water. The oil is poured off, the break and water are discarded. To save as much of the oil as possible, put the last bit into a tall, narrow container and let it sit until the oil has again come to the surface, then return it to the jug. The process is repeated a total of five or six times, all but the last time containing sand and salt. The last wash is simply water and oil, the water changed several times to make sure all the salt is removed.

      This oil is slightly cloudy when it is finished due to residual water in the oil. There are several traditional ways to clear it, including the addition of a small amount of calcium oxide -- quicklime -- the addition of wood chips such as those used for pet bedding, breadcrumbs, or simply leaving it in a warm windowsill for a time. I've also found that a small addition of lead carbonate clears it quickly. But it is simplest to clear the oil by heating it slowly and carefully, with occasional stirring. Oil which still remains cloudy can be frozen overnight, it will then clear in a sunny windowsill.

      This method substitutes the mixture of diatomaceous earth and cristobalite used to filter swimming pools for the sand in the older method. The material I've used can be ordered here in the USA. The diatomaceous earth binds to the fatty acids, the cristobalite acts like the sand in the older method. This mixture is relatively fine but in summer still falls out of the water quickly. In winter a low oven is needed. This material takes the break out of the oil much faster than using sand. Photo below shows jar after second washing, the break has actually fallen in huge clumps to the bottom of the jar instead of clinging to the oil. Proportions used in this test were: First wash: 2 cups oil, 4 cups water, 1/2 cup filter mix, 4 T sea salt. Second wash is the same. Both diatomaceous earth and cristobalite are forms of nearly pure silica. It is important to wear a particle mask when working with this material in the dry form. This procedure does not work with food grade diatomaceous earth and sand as the diatomaceous earth has not been calcined first.

      Oil after the first wash of one day. This was placed in a low oven to create the separation of oil and break. Note the large amount of break, about twenty percent. Because of being physically bound to the diatomaceous earth, some of it has fallen to the bottom of the jar. The break is composed of water-soluble fatty acids which are known to be a potential source of both short term and long term yellowing, especially linoleic acid or Omega 6. This will be washed two more times and rinsed with water before being placed in glass jars in the light to age.

       You can download a pdf file with the latest version of this process and several traditional variations here.

      Various finished versions of this oil aging in the light. The alternation of cold and sun, and the addition of a small amount of chalk or lime tend to precipitate out further small particles of break. One would think that the lightest oil would be "the best" but this is not necessarily so. For example, I'm very interested in the far right jar of the first row because it's combination of post-wash additives has produced the most break, it's still coming out after a month. So, conceivably, it will be the most fatty acid free, and therefore the least likely to yellow over time.

      Recent yellowing test (January 2010) of various organic cold-pressed linseed oils refined by the methods here. It will take several months to see the full story, but I've never had linseed oil dry this close to colorless before.

>      Beyond the yellowing test, but quicker to show potential long term issues, is a stress test. In this case, samples of the oils were places on white enamel jar lids and baked in a 225F oven for four hours. Some overall yellowing, but not at all bad. A bit of a surprise in the lower row, middle. However, a commercial oil.

      There are several types of walnut oil, from the ultra-refined oil sold by Kremer to the mechanically refined but cold-pressed Spectrum Naturals oil, to the unrefined Kremer or Loriva brand oil. The Loriva oil has recently been altered to include "walnut flavor concentrate" and should probably be avoided for this reason. There are also relatively expensive walnut oils from France or England, good if you can find them in a dented can store, but beware of any oil containing a preservative, vitamin E, as this will retard drying completely. In the unprocessed state, the most crisp paint is made by the unrefined oil, but although I haven't experienced any short term darkening from unrefined oil it does take longer to dry and this can be a factor in yellowing. I've chosen to work principally with Spectrum Naturals, which is in the middle both in terms of color and working character, and wash it. For the most part I use this oil in an "aged" or possibly "semi heat-bodied" state, see below, another older technology which is simple but very helpful: makes for less yellowing and a faster-drying paint with more spring or "boing".

      The washed, preheated Spectrum naturals oil can also be added to a lead tray to make a thicker oil with a different rheology than sun or stand oil, see below.

       Typically, oil is heat-bodied by "fuming," heating just to the point of smoking, for a period of up to an hour. This produces a darker and much more resinated oil which is somewhat slippery, though still more fluid than sun oil or stand oil. Using the repeated preference in older texts for "old" oil as a clue, I found that by lowering the temperature to around 230 degrees Fahrenheit (100C, the boiling point of water) another level of possibilities emerged as the oil at this temperature doesn't darken and becomes only slightly thicker. This is an area where a great deal of latitude exists: I've experimented with heating for as long as ten days, but the time that I work with most in making paint is 48 hours, mixing this with raw oil in different proportions. This 48:230 oil is also a good foundation for mediums. It is possible, if no means of maintaining a low temperature over this long period exists, to raise the temperature to 300 Fahrenheit (about 150C) and stir the oil for an hour or so. This is still well under the temperature (280C -- 536F) at which the triglycerides begins to break down.

      Paint made with preheated walnut oil and raw sienna, no other additions. This paint is inherently thicker and more bouncy, without becoming glutinous or stringy. This approach offers many fascinating rheological possibilities using the standard array of inert additions and medium ingredients. It was interesting to note this paint's increased sensitivity to the introduction of a hard resin varnish. While amber, copal, or sandarac will cause most commercial tube paint to become variously more thixotropic, this characteristic of seizing or gelling becomes significantly more pronounced and reliable in homemade paint. When using a preheated oil paint any hard-resin addition must be measured very carefully as the paint becomes so immobile that solvent or more oil must be added at very small concentrations. Eastlake gives very early evidence from the Strassbourg MS that paint was made with oil that had been boiled briefly with a drier, in this case calcined bones and calcined zinc sulphate. This same passage (15th century, information from 14th century) also instructs that the paint is to mixed with a few drops of varnish on the palette. There are many other examples from older texts of oil heated with or without driers prior to use. This oil holds the pigment in suspension better than raw oil, and the double-carbon bonds between the triglycerides formed by the heat create a different type of ultimate polymerization than that of raw oil: atmospheric oxygen is less involved, resulting in a decreased potential for non-reversible yellowing in the paint film by hydroperoxides. So, whatever purification process is or isn't used on cold-pressed, unrefined linseed/walnut oil, a final stage of heat prepolymerization seems potentially as important in attempting to minimize long term yellowing. In my experience this oil has been superior in every way to raw oil -- although I haven't experienced raw oil aged more than a year, an item older painters may very well have had access to.

       This oil can most effectively be made using a magnetic laboratory hotplate, but can also be made using a small deep-fryer with a thermostat or, hotter but still acceptable, the small Crockpot made for heating "party dip". This oil also became the foundation of the chalk and oil putty mediums, with small amounts of thicker oils in various recipes. All these preheated oils dry harder, more quickly, and more stably than raw oil without the deeper saturation, shine, and dramatic change in rheology of fumed or sun oil.

      Mixture of Burnt Green Earth and Dark Ochre made with preheated walnut oil and nothing else. Dense but mobile paint like this cannot be made on modern machines without significant additives. When older painters talk longingly of older paint, it is because it was made with more pigment. Even high quality paint from the 1950's -- Lefebvre-Foinet, or Windsor and Newton in the old style tubes -- has this more dense pigmentation.

      A material which is capable of many, many rheological behaviors is lead carbonate, an entire book could be written about this alone. It doesn't have to be from the stack process to be worthy of your time. Always wear gloves and a serious particle mask if working with toxic materials in powder form.

      "Heat pre-polymerization has several effects on the oil. Drying properties are improved and are further enhanced by the addition of metal salts (usually those of lead) during the process. The refractive index of the oil is increased, thus reducing light scattering at the pigment-medium interface and thereby increasing the saturation of the pigment colour; the paint film may also have a glossier appearance. The pigment is less liable to sink in the oil film, which itself decreases less in volume than a conventional oil film, reducing the amount of wrinkling that may occur. White paints appear less discoloured because, as the polyunsaturated fatty acids initially present in the paint film are destroyed by the formation of carbon-carbon single bonds, there is less scope for the formation of chromophoric and auxochemic groups, the presence of which give the yellow appearance to the film." From Rembrandt and his Circle: Seventeenth-Century Dutch Paint Media Re-Examined by Raymond White and Jo Kirby. National Gallery Technical Bulletin volume 15.

      It would be logical for an oil rich in carbon-carbon bonds to also be more resistant to subsequent oxidation of the paint film. But there is of course some question as to just what "heat pre-polymerization" means in this case.

      Using painter-refined, cold-pressed organic linseed oil, many pigments available now can simply be mixed in needed amounts on the palette. While not as saturated or fine as paint made by mulling, this is a good way to begin to understand the handmade paint process. Pictured, Burgundy Red Ochre Deep, No. 11576, available from Kremer Pigments.

      The paint is naturally somewhat dense and stringy.

      Separating the paint before mixing with a chalk putty medium.

      When mixed with a chalk putty medium, it becomes lighter, but also brighter.

      If a denser texture is needed, more chalk can be added.

      Many different values and rheologies are possible without resorting to white or a medium containing resin or solvent.

      The older books contain a great many different recipes for making oil dry faster by a process of mixing a metallic oxide with the oil. The ingredients mentioned most often in the 17th century texts are lead carbonate (white lead) and lead oxide (litharge), replacing the calcined bones and calcined white copperas of earlier texts. Lead acetate is mentioned by Eastlake and comes into play in the 19th century recipes for collected by Carlyle, but is not mentioned specifically in de Mayerne, or any of the older texts in Merrifield. Lead acetate, however, was present in variable quantities in lead carbonate made by the original stack process of exposing metallic lead to the fumes of strong vinegar. This is of potential importance because the gelling nature of the acetate in oil creates the possibility of creating a gelled medium without the use of mastic. Lead acetate, however, is very poisonous.

      A very simple method of making a drying oil involves placing a small amount of lead carbonate in the oil and placing it in the sun for a time. Faster would be putting the oil-lead mix in a water-bath and boiling for a given amount of time: this is the method reported by De Mayerne as Van Dyck's: it certainly results in a limpid product. Litharge can be introduced, which creates a more drying oil with more initial color, but the color does depart as the oil dries. If the oil containing litharge is boiled, this results in the material called Black Oil, which can be compromised in terms of film strength by prolonged heat and saponification. The type of drying oil recipe I've worked with the most incorporates a small amount of water into the heating process: this concept is mentioned in Eastlake, as well as several of the older texts in Merrifield. Oil made this way is slightly saponified, it will foam under the brush, and relatively immobile or adhesive while still being quite liquid. It also dries quite soft and flexibly, making it a logical partner for a hard resin varnish, for underlayers which will ultimately be protected, or on stretched canvas. All leaded oils will continue to react to oxygen and will throw a deposit of lead soap as they age and clarify. To keep a leaded oil thin oxygen should be excluded from the bottle. Leaded oil can also be purposely allowed to thicken for specific broken surface impasto techniques, is less pervasively sticky than sun oil; these can of course be mixed. Another interesting technique is to allow some of this oil to thicken into taffy, then blend it with a bit of fresh oil, giving the body of the thickened oil with the movement of the new oil, not unlike a gel medium, but without the use of resin.

      Leaded oil is now a controversial material, perhaps because, like many things which worked in a small amount when painters understood the craft, later less technically aware painters assumed that a larger amount would be better. This becomes very clear during the extraordinary "lead acetate phase" in England during the 19th century so well documented by Carlyle. Making a leaded oil with a relatively small proportion of lead to oil is more than sufficient to get a layer of paint to dry overnight when used in a putty medium or with a hard-resin varnish. The simple lead carbonate oil is the least altered, but also offers no increase in density or adhesion: great, however, for developing very large portrait canvases quickly. The water-modified oil offers more thixotropic potential at the expense of some strength, but this can be balanced by a small addition of resin varnish, sun oil, or a stone dust putty which again increases the thixotropic, broken-surface potential of the paint.

      A simple, low-tech method of making a leaded oil is to put the oil in a tray made from a sheet of lead, which can be simply folded by hand for this purpose: use gloves, works better with older -- oxidized -- lead than a new sheet. There are many older recipes calling for sun-oil to by made in a lead tray. The oil can remain in the tray until any desired degree of thickness is obtained: a full tray will not thicken as quickly as a thin layer of oil, but the oil will dry faster than oil that hasn't been treated even if it appears ostensibly the same. The oil obtained over time from a thin layer will be somewhat long and gluey, but not sticky, or as taffy-like and thixotropic as the recipe below. However, it is stronger and dries harder than Eminent Oil. This oil is also bleached by the process. In the first few batches this oil becomes cloudy, but still dries clear. As the lead is used, the newer oil added doesn't cloud, but still thickens significantly. This process does take several months to produce something of a thickness analogous to stand oil. However, here the similarity ends. Cloudy oil can be cleared more quickly with a small addition of spike. The cleaner the oil is to begin with, the less likely it is to cloud. For more about this oil go here.

      This is adapted from Eastlake, who attributes it to an eminent painter, recently deceased, who used an old Flemish recipe to make it. (There are also similar procedures in Maroger which he got from Merrimee, but containing more lead, see below. This concept of adding water also shows up in a couple of texts in Merrifield, Lebrun and a note from and 18th century Italian text. ) As is often the case, the proportions of the original are sketchy but I did the following, cross-referencing this technique with recipes in De Mayerne: For two cups oil, added 1.5 to2.5 ounces water (more water, more saponification of the oil) and 1T each of litharge and lead carbonate, grinding well to incorporate the solids. (Both of these are TOXIC, please use all appropriate precautions when using). This was then heated -- outside -- to the point of the water boiling and let go until the water evaporated, about an hour. You can tell when it's done because the bubbles and slight crackling sounds stop. This can foam over very quickly once it gets started so use a LARGE pan and be very watchful in the beginning and lower the heat quickly or remove the pan from the heat if you begin to see the foam rising with increased speed. It will keep going, trust me on this, unless you cool things down quickly. So, use the lowest heat that will keep it going, stir it but not with a wooden spoon, which creates increased foam. After the foam changes from orange to a pale, tawny beige, it's okay to turn up the heat more, but it needs to come off the heat as soon as the bubbling and crackling sounds stop to avoid getting dark. This throws off a reasonable amount of lead soap as it sits and takes time to clear (some will be available to use the next day, the majority clears in a week or so) but is much lighter in color than an oil made without water. While it is tempting to use it right away it is much better in the long run to bottle it and place it in a sunny place. In a few weeks the lead salts will fall to the bottom of the bottle, leaving an oxygenated and slightly grippy oil with a minimum of lead remaining in it. Please DO NOT under any circumstances make this with litharge alone in an effort to make a lighter "black" oil, as it can cause major spattering of the oil in the latter part of the process due to trapped pockets of steam. Maroger himself discusses this danger, this recipe track evolved with both forms of lead for a reason.

      I've been used this now since 2004 and feel positive about it, as long as it is used in moderation in thin layers, and allowed to age before use. It is perhaps most useful as a couch or medium for the beginning layers of the painting. When made correctly, this oil has little color, and when left exposed to air, loses all color: Eastlake documents several historical sources which point out that leaded oil becomes transparent on exposure to light and air. It also begins to thicken, slowly but surely, until it is a dense and pliable taffy, opening up the door to a wide range of broken surface techniques. People have reported using this as a substitute for Black Oil when making Maroger medium. There are a couple long term issues with Maroger medium -- yellowing and the attack of a soft resin by oxygen -- and if I were to go this route I'd make Roberson's medium (recipe is here, scroll down), one of the mediums of the PR Brotherhood, which contains copal in addition to mastic and was whipped for a long time to ensure fusion. I tend to use this oil in a mix with heat bodied but unleaded oil, or in conjunction with a small amount of resin varnish: the leaded oil is soft and flexible, the resin hard and inflexible, together they make a logical team.

      If eminent oil is exposed to the air for a week or two it becomes very long and ropey, does not skin over, and will ultimately become a clear taffy. This can be used instead of sun oil as a starting point for an impasto medium with ground leaded glass or other inert additions. It can be used as a kind of starter for thickening oil. It can be allowed to set firmly, then cut again with fresh oil, resulting in a material that is not long or ropey but both dense and mobile, with many of the qualities of a gel medium. When made with walnut oil, may yellow slightly at first, not after aging in the light.

      Recently made Eminent oil settling out. Bottom layer is the lead solids. Middle layer is water-soluble fatty acid "break" being released from the semi-refined walnut oil used in the process. As it ages this will continue to throw a precipitate of break: the aged oil yellows less as a result.

      This oil can be used to thin the paint in the later layers, is perhaps not rich enough to finish a painting, but close. It's composed of one part Eminent Oil and three parts oil which has been heated to 150 C for four hours. The two oils are mixed thoroughly and given ten minutes in a boiling waterbath. This oil is relatively thin but somewhat adhesive, it doesn't run. It becomes increasingly thixotropic and loses all color on exposure to oxygen.

      Fumed, pyrogenic, or colloidal silica is a pure form of silicon dioxide made at very high temperatures with various industrial uses. It is chemically the same material as quartz. When oil is combined with this material it forms a gel. Various viscosities of oil can be used and various ultimate viscosities of gel can be achieved. Works well for both lively alla prima work and as a thin couch in later layers in more formal painting. The key here is knowing the behavior of your oils well and using as little of this medium as will do the job effectively. Vastly preferable to the fragile mastic gel mediums, can be made to perform in a similar way. Fumed silica is very light and needs to be handled with care using a respirator. The gel can be made in bulk and stored in tubes or wrapped airtight in aluminum foil. If you make this, try some small studies and tests first to get an idea of how it handles before making larger amounts. This material needs to be made with an oil component that dries well. Fumed silica is very light and responsive to static electricity, great care should always be used opening the tub. It is a respiratory hazard and should only be handled in the dry state wearing a quality particle mask.

      At the far end of the thickness spectrum is the sun oil of the older painters, can be made with any drying oil. This should be a cold-pressed oil, it should be mechanically refined or washed to remove the fatty acid break but should not be the alkali refined commercial oil. I've had experience of sun oil made with unrefined oil darkening. You need a hot dry climate to do this efficiently. The oil goes into a glass tray, a lid of glass goes on top but with an air space for ventilation. Several older sources talk about doing this in a lead tray or adding pieces of lead or adding a small amount of lead carbonate. This is a faster process but also results in a somewhat different material. Stir it now and then to make sure no skin is forming on top. Or allow one to form that is thick, and simply cut through it at the end. Heat gently to thin at the end in order to strain out bugs and other foreign matter. This takes about a month in a good climate, longer with less sun. However, even two weeks in August in Vermont, while not resulting in a particularly thick oil, does result in a pronounced increase in gloss and decrease in drying time. If you live in a more ideal climate don't let it get too thick as it continues to polymerize after being removed from the sun. A small amount of this is useful in mediums and is vastly superior to commercial stand oil, which dries slowly and can yellow badly. Sun oil is lighter and more sticky than fumed oil.

      Later in his career, Lord Leighton stopped using Roberson's medium and began using Bell's medium, made from blown oil and spike lavender. A perhaps more high quality version would be to use sun oil for the blown oil. A good place to start with this for use in a medium would be one part spike lavender and one part sun oil, using about 25 percent of this mix with other, thinner oil(s). For use alone, sparingly, two parts spike to one of sun oil could be used over a developed underpainting, or three spike to one sun for more liberal use. In finishing, the one to one mixture is fine, but should again be used sparingly. This has an open time of about half an hour, then sets progressively like a resinous medium. Spike lavender is very penetrating, and should always be used with ventilation. In the yellowing tests I've conducted, this medium yellows minimally. Should be kept in amber glass, small bottles.

      There are many sun spike variations. This one is perhaps the best all-purpose, non-resin thin painting medium I've made: dries hard, moderate shine, a thin layer dries overnight in summer. Can be used thinly as a couch in layered techniques, more open time than the original sun-spike but dries as quickly. 50ml sun walnut oil, 70ml spike lavender, and 20ml Eminent Oil, see recipe above. Combine and heat in a waterbath for ten minutes. While spike lavender may not smell bad it is still a strong solvent and needs to be used with adequate ventilation.

      The original burnt plate oil was made by heating linseed oil, with or without a metallic drier, until it combusted, stirring until the material achieved a highly stringy and elastic quality. This material has been proposed by conservators Sarah Belchetz-Swenson and Phoebe Dent Weil as the material used by Rembrandt to make specialty impasto white: the process is illustrated here. While the extensive use of heat-bodied oil is not suggested by the research done by London's National Gallery and documented in their book, "Rembrandt: Art in the Making", this is nonetheless an intriguing material, in that the process results in a very long, highly saturating oil which is not sticky or glutinous like stand oil or sun oil. Graphic Chemical Company makes a line of linseed oils in various viscosities that have been cooked in a vacuum kettle at hight heat, with the volatile elements being progressively sparked off. A very small amount of one of these oils in a medium changes both rheology and drying time significantly. Oil #3 is thinner than stand oil, and has a bit of color. Oil #5 is about like stand oil, but is very pale. Oil #7, illustrated, is thicker than stand oil and is also a pale yellow. Oil #8 is like cold molasses or treacle, and has a light caramel color. These oils seem to dry to their color with minimum subsequent yellowing. However, they do take a long time to dry and increase the drying time of a medium. I've used very small amounts of #5 in a finishing medium, and have incorporated both #5 and #7 in small amounts into putty mediums. These oils in a medium produce paint which is ductile and blendable while retaining body and with significantly enhanced saturation. As an inexpensive commercial product available by the gallon, I was highly skeptical of this material, but it has proved itself in practice to be useful in very small amounts.

      The putty medium is simple but can be configured in many ways. It will make paint which is thin and smooth, paint which is rough and broken, and everything in-between. It came about from studying the book "Rembrandt: Art in the Making" by the National Gallery Staff, in which they site research at a molecular level which determined that Rembrandt principally used linseed oil and chalk as the additions to the pigment. Paintings on this site since 2007 were executed in variations of the putty medium. More putty process details to be found here. More putty history and instructional text to be found here.