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A Meguilp by any other name....
Macgelph was/is a jelled compound of spirit mastic resin varnish and linseed-based drying-oil. This oil painting medium was likely first used in the years from the later 1600's, though its first known reference dates from about 1760. The "jelly" became especially popular through the next century. An even earlier incarnation based upon walnut-based drying-oil was used in 1600's Italy (see Bombelli on our Varnishes & Mediums page). I should add that throughout its recorded use in historical technique, Megelp has had reputations ranging from very good to very bad. It's use is typically condemned by our currrent crop of experts and conservators; however, too many first-rate oil painters, such as John S. Sargent, couldn't seem to perform without the gelled medium ...and, on the whole, their own favored use has seemingly engendered no untoward problems.
As there is much confusion today as to what constitutes the infamous Meguilp, my cause for developing
this essay was to shine a light upon the matter of its identification or chemical make-up. In
order to accurately focus this light, I have gathered together only original formulas from primary
historical sources , that is, recipes and formulae from the actual historical time of Meguilp's
Of course, today's widespread use of "Maroger's Medium"-- a jelly medium much favored and
contested-- has a perfect place within this matter. There are today those who maintain devoutly
the "Maroger's Medium" is not actually Megelp and that the Macgellup is something else
entirely. [As you have now noted, because of its once wide-spread usage, Macguilph has
In the 1st paragraph of introduction to his 1948 book, "The Secret Formulas and Technique of
the Masters", Jacques Maroger wrote: "Ever since the knowledge of the great painting
techniques of the Renaissance was so mysteriously lost, about the end of the seventeenth
century, artists have been vainly trying to rediscover the methods of such masters as Titian,
Rubens, ....as well as their predecessors-- Jan Van Eyck, Memling,......whose
techniques...allowing...facility of execution...were... brilliant and durable."
In his second introductory paragraph, Maroger adds: "It has become evident to every artist who
has worked in the medium of oil paint, for the last two centuries or more, that certain qualities of
color and modeling and brilliance of surface which seem to have been the common possession
of earlier schools of painting, were, with the resources currently at their disposal, completely
beyond their power to recapture."
Maroger's lengthy but nicely worded intro certainly would find agreement among many learning
or practicing painters today...as well as times long gone. As example, and not suprisingly, one
hundred years earlier, the same lament was being delivered within a now well-known book by
Charles L. Eastlake, "The Methods and Materials of the Great Schools and Masters", published
in 1847. However, it could be well argued the circumstances that introduced each book were,
amazingly, 180 degrees opposed to each other.
On page 100 of Maroger's book, that author describes what he believed was the medium of
Rubens. He called it "Rubens' Jelly" and this particular formula would, more than any other
medium he presented, become known and much lauded afterwards as "Maroger's Medium". To be
precise, here are Maroger's own words: "In general, a spoonful of mastic varnish, mixed with a
little more than a spoonful of black oil, gives a good result (jelly)." On page 177, Maroger gives
proportional directions for the mastic varnish for making the jelly medium as 1 part mastic resin
(measured by weight) to two parts turpentine (also measured by weight). This makes for a strong
though still thin varnish. Also given, elsewhere, are formulae for the well-known "Black Oil",
made by heating small amounts of either Lead Carbonate or Litharge into the linseed oil, giving
the oil the color of coffee as well as making it dry rather quickly. [Maroger,p.64, attributes the
formulation of "Black Oil" to the painter Giorgione]
Maroger goes on to remark, page 100:
"A similar, more recent, jelly has been known under the
name of Megilp, or painter's butter, but this concoction, as it has come down to us, is so
deformed that it no longer bears any relation to the jelly of Rubens. It is not even usable on
account of its too great siccativity and one can say that it is to the original jelly what siccative de
Courtrai is to the Black oil-- that is, a product completely transformed, retaining only the name
of what it formerly was."
Curiously, though his book is filled with recipes, Maroger did not provide a single formula for
the "unusable" and too siccative Megilp. I want to say herein that there are many interesting and
valuable assertions to be found within Maroger's book, and he often supplies useful or insightful
information. Yet, nothing more was writen and it seems the particular and supposed distinction--whatever it was-- between Maroger's "Ruben's Jelly" and the common 18th-19th century Megilp jelly has caused much confusion among painters ever since.
Maroger's book was printed in 1948, and some might believe books on Oil Painting were unique
to the 20th century. Of course, this is not so; there has never been a dearth of artists seeking
either remuneration, or the stature of teacher, or perhaps nothing more than idolatry, in exchange
for their learned experience. Aside to and building upon the earliest recorded manuscripts, "how-to" instructional tomes about art have been "out there" for centuries. Cennini's "I'l Libro Dell'
artes" came down to us from the Renaissance. With each generation, and compounded through
each century, more have followed.
Historically, the jelly painting medium known as Megilp (or its many alternate names) began to
appear in notes and writings along about the mid-18th century. For example, Reynolds first
mentioned the oddly-named substance in his notebook of 1767. After that time, the jelly stuff
seems to leap easily and often into printed allusion.
In fact, many how-to oil painting books written during the later 1700's and well into the 1800's
mention Megilp as a useful medium. Typically, the various authors call for the mixing of
"Drying Oil" with a quantity of "Mastic Varnish." For instance, Eastlake mentions on page 310
of his "Methods and Materials..." , 1847, vol.1, that Megilp is composed of "drying oil and
This may cause some confusion as one might fairly wonder what is meant by the terminology of
"drying oil" and "mastic varnish". For example, did "drying oil" mean merely raw oil? In today's
nomenclature, it certainly does...but what was meant by this terminolgy in the past centuries?
And what goes into the make-up of Mastic varnish? Thankfully, the various historical "how to"
painting books from these earlier and Victorian time periods are actually specific, though not
always specific in the same sentence. Upon reading further within these sources, we do often
find the ingredients clearly defined. To wit, a perusal of the painting manuals shows a definite
trend; "drying oil", as concocted before the acceptance of Manganese in the later 19th century, is
typically made by the treatment of raw linseed oil with either lead or a compound of lead--
usually lead white, red lead, or litharge. As for the mastic Varnish, it is formed by dissolving
Mastic resin with Essential Oil of Turpentine (today's typical Gum Turpentine).
Artists have lately become aware of the recent and welcome research into historical oil painting
mediums by Canada's Doctor Leslie Carlyle. To much credit and value, Carlyle has generated
her historical research in a common sense and very practical way; her method was /is to seek
the "horse's mouth", i.e., exploring actual painting literature from the time periods.
On page 54 of her 1991 doctoral dissertation, "A Critical Analysis of Artist's Handbooks,
Manuals and Treatises on Oil Painting Published in Britain Between 1800-1900: with Reference
to Selected 18th Century Sources", Carlyle summarizes a description of drying oil under the
subject heading of "Methods for Preparing Drying Oil": Dr. Carlyle writes: " Drying oil was
prepared by partially polymerising the oil either by exposure to light (sun-thickened), or heat
(boiled oil), or by one or both of these processes in conjunction with "driers". "
To the careful eye, this encompassing statement allows much for ambiguity, and is certain to be
used as the reader may capriciously deem fit. However, Dr. Carlyle goes on to record the
widespread use of Lead (and it's oxides, sulfates, and acetates) as the most commonly-used drier
for making commercial or home-made drying oil until the later part of the 19th century. In
almost every 18th and 19th century book source alluded to in Carlyle's paper, lead in some form
is mentioned as the drier used in making the drying oil. As for "Mastic Varnish", Carlyle cites
and shares the recipes of three separate 19th century authors, Ibettson, Field and Williams;
plainly, this varnish, depending upon final use as a protective coating, or as an adjunct to a
painting medium, was made by either a thin or strong combination of Turpentine heated at low
temperature with Mastic resin.
Additionally, that very popular 18th & 19th century medium known as "Megilp" is also covered
well by Carlyle and her lengthy paper provides recipes copied word-for-word from the numerous
19th century 'how-to' painting books. There are subtle differences between authors but a certain
enlightening commonality prevails. I will supply a few of these as sample and in somewhat of a
geographic spread to show the same general knowledge was widespread at the time:
In England; J.C. Ibettson, "An Accidence, or Gamut, of painting in Oil", 1803, page 15; under
the heading of "Megilp"... "The very best pale is made by boiling the linseed oil in an earthen
pan, at the bottom of which white lead is spread a quarter of an inch thick; do not stir it at all
until it turns a brown ash colour, when it will have imbibed a sufficient quantity of lead to turn
the mastic varnish and itself into a stiff jelly."
Another: In France, 1827, P. L. Bouvier wrote a book for beginning painters (Manual Des Jeune
Artistes...") and included the following: Name: Macgilp, Maguilp, Magelp, Magilp; Materials:
Linseed oil boiled on Litharge; strong mastic varnish; materials ratio: 1:1 or 2:1
Some years later, in 1845, an American, L. Osborn, translated Bouvier's work with additions and
had it published in Philadelphia. According to his book, having the same title as Bouvier,
"Handbook of Young Artists and Amateurs in Oil Painting", 1845; Chapter 8, page 79 :
"Macgilp, Maguilp, Magelp, or Magilp (for we find this odd word spelled in all these ways).
This preparation, whose name is derived we know not whence, is for sale at the colorshops. It
consists of linseed oil boiled on Litharge and mixed, by simple shaking of the phial, with half, or
an equal quantity of strong mastic-varnish. It has of course, considerable body. It is much used
by some painters and chiefly for retouching."
Another 19th century author, whom I have seen mentioned often in the personal notebooks of
American artist William Sidney Mount (a well-known New York portrait and genre painter), was G. Field,
who wrote "Chromotography: Or a Treatise on Colors and Pigments", 1835. In the realm of
making Megilp, Field wrote: ".... the simple solution of Mastic in rectified turpentine, which is
easily prepared, by digesting in a bottle during a few hours, in a warm place, one part of the dry
picked resin with two parts or more of the oil of turpentine. One part of this, cleared, varnish
combined with two of either of the before-mentioned drying oils of linseed, more or less
according to the purpose of the artist, constitutes the transparent Macgilp of the painter (page
206)." On page 205, Field refers to the two types of drying oil: the former is prepared without
strong heat using a lead compound, the later is boiled with Lead.
Another: Clint (1855), page 64; Under the heading of Brown Megilp, this author writes: "Brown
Megilp is composed of equal quantities of strong Drying Oil and strong Mastic Varnish, well
shaken or stirred together. If a small quantity only be required, put a given number of drops of
each on the palette, and rub them well together with a knife, when they will produce a strong
transparent jelly; this is an old-fashioned vehicle, but in my opinion a very good one."
For accuracy, there were some slight historical variations also known as Meguilp. But, a thorough look reveals all such recipes realize the basic necessity of combining Mastic in turpentine varnish with some sort of drying oil made with lead. Also known from the late 1700's is another slightly differing concoction known as "Gumption". An invention of the aforementioned Julius C. Ibettson, Gumption utilized all the basic ingredients except the turpentine. The Mastic resin was mechanically ground up into the linseed oil, a small amount of Lead Acetate was required and the yellowish opaque glop did form into a stiff jelly. More oil was added until a favorite consistency was achieved. Some tests show Gumption to be a better lasting medium than Meguilp.
To summarize, when I compare the basic 19th Century Meguilp formulas with J.
Maroger's 1948 "Black Oil" and "Ruben's Jelly" formulae, my own mind cannot but see the very
same and exact product cloaked within the guise of another title. Further, it must dawn on me
that Megilp undeniably became popular and certainly used by some of the greatest and skilled
painters throughout the 18th and 19th centuries, a time when Maroger claimed the secret to great
technique and brilliance in execution of the previous centuries (15th, 16th, 17th) was, in his own
introductory words, "mysteriously lost". His book strongly advocated the use of his mediums as
being those of the old masters; for repetitive instance, according to Maroger, Georgione used the
"Black Oil" and Rubens used the "Mastic Jelly". Realize, Maroger urged the use of these same
mediums as sure routes to returning the lost Renaissance magnificence to today's practicing
Thus, beginning at mid-20th century, Maroger railed against the painter's practice and use of the hard resins, Amber and Copal, then in broad and good use by great painters from 1850 to 1920 (a practice in great part due to the worthy work of Charles L. Eastlake whose own book was first published almost exactly one hundred years before Maroger's). However, as the numerous 19th century introductory painting manuals attest, these same "Maroger Mediums" were actually well-known and commonly used throughout a span of time which Maroger detested and decried as lacking those meritable painting accomplishments attained during the venerated "Old Master's" time-period.
USE of MEGUILP and OTHER GELS in PAINTING
Below is an image comparing a commercial artist's best CP linseed oil, foots-separated unrefined walnut oil, and foots-separated flaxseed oil. All oil paint piles were compounded exactly the same way with lead carbonate and placed flat in a daylight-lit room. The nut oil in the center dried the fastest (4 days) and did not collapse into itself, wrinkle, nor turn yellow, as did both types of linseed oil. This is a simple but compelling illustration of the relative effects different oils, alone, have upon artists oil paint. No mastic was involved with this test; and yet faults such as these have been historically attributed to using mastic megilp. As this shows, potential problems inherent to the type of oil involved in the paint and medium have been entirely overlooked.
If anyone cares, I suspect the Megilp (and other gel-oils) to be a useful medium when added to oil paint in small amounts and knowing technique. After all, there are what I consider "great" 18th and 19th century works yet extant and known to be made with this material. Perhaps, as some say, the condition of these works is in great part due to the skills of restoration, or other means-- I do not know.
Photo shows two gel-mediums of my own manufacture not based on wax or Mastic resin.
The plus sides to using Meguilp as additions to paint are: it engenders faster drying of the paint, it allows a free sketchy quality in rub-ins or ebauche monochromes or multi-chromes. The following examples show the effect of using a gel-medium in this manner ( not Meguilp but a gel medium of my own make which contains no Mastic or wax).
The gel medium was rubbed thinly over the non-absorbent white ground and a brown (made from black and indian red) was quickly scrubbed over the whole, resulting in this rather defined monochrome. To continue the work, after drying, the ebauche can be rubbed over with another layer of the gel medium and colors superimposed without harming the initial ebauche...sort of like coloring a B&W photograph. Be aware Meguilp can cause problems at this injunction as it contains turpentine which can dissolve away some of the ebauche detail. Here is a close-up showing the effect of using the gel medium:
For contrast, here is a simple ebauche monochrome made without a gel-medium, and, instead, a thin application of drying oil as the lubricant:
A fair amount of detail is preserved by use of the drying oil, and it does dry quickly. As it is a non-gel, a somewhat "softer" appearance results. Still, such similarities can confound viewers as regards the actual means taken to get these effects. However, the seasoned eye can often pick these nuances apart; but not always.
Having spent much of my life studying the Hudson River School, I have little doubt there was much use by these painters of either a wax-type medium, Meguilp, or the version of Meguilp with Copal varnish-added-- a mixture known as Roberson's Medium. As example, the typical sketchy "oil-icious" underpainting in browns so effectively utilized by F.E. Church (especially in his 1850's, '60's originals, sketches, and studies) seem to indicate the use of a fast-drying jelly medium which captured the hatchings of the soft brushes near perfectly. Either while still wet or later, after drying, this hatching monochrome was then painted over with colors.
This peculiar effect of 'keeping' brush hatchings has appeared time and again since the 1400's. I have found that a smooth un-absorbent lead white or other oil-white ground is almost a necessity for the occurance. A few other routes can be had. For instance, a gesso ground sealed by a coat of glue or shellac ; and especially if it is also coated with a thin layer of drying oil, then seasoned a bit before use. Combined with such a ground, a jelly medium shines in its performance. By comparison, a modern-day acrylic gesso ground fails with all but a firmer wax-involved medium.
But, again, as regards mediums, there are other decidedly more effective routes besides megilp to creating such hatchings on a smooth lead white ground, such as the common 19th century practice of adding a little wax to the medium used for lubrication and paint. And, if it is made using drying oil, such a medium would also dry quickly and perform like Meguilp. [Regarding Church, his teacher, Thomas Cole, also produced the same jelly-like effects in many of his own later works ; and the best historical clues say the "Father of the Hudson River School" used a particular sort of copal varnish ; plausibly one that inherently allowed a 'keeping' effect when simply added to raw oil. For good example, the simple 1:1:1, turps/ oil/ varnish medium so popular back then, can be quite effective if a bit of lead white is also added to the color to be scrubbed upon the oil-primed ground ; the results appearing exactly like wax or megilp had been incorporated (in fact, all sorts of wonderful painting effects can be had by such means combined with a brush lightly dipped in solvent from time-to-time and going back into the applied wet paint). Thus, as it stands now, I am inclined to think Church may have achieved his own jelly-like effects without resorting to megilp ; though I do reserve suspicions he and others may have added some wax to their little concoctions..... If anyone has any information about Church's painting mediums, please send me a note.]
In summary, the Megilp is a historical medium with a "colorful" history and I respect it as such; yes, it is
interesting stuff. Of course, Megilp is based on the need for Mastic resin-- certainly one of the softest natural resins known to painting use. This soft resin can sometimes make up about 1/4th the actual medium (after turps evaporation) and so can allow the dried Meguilp layer to remain dissolvable by the weaker solvents. Of course, when Meguilp jelly is used as a means for initial ebauche, the entire monochrome should be covered (sealed) by additional painting wherein the amount of Meguilp is kept proportionally low; otherwise, the pure Meguilp lubricant-layer will remain exposed to the air and future cleaning solvent attack
My thanks to James Morton, first-rate Oil Painter (and oil painting historian) from Colombus, Ohio, for his generous and thoughtful sharing of Dr. Carlyle's dissertation.
Dr. Carlyle's compilation and study of the various instruction manuals makes for enlightening reading. I understand her dissertation may soon go to press and will be available to bookstores in the near future.
Precautions Concerning Use of Jelly Mediums
All jelly mediums, synthetic and natural, suffer problems due to overuse. Though many agents often get blamed , the party MOST guilty for this offense is undeniably simple: the oil, and especially linseed oil.
I know this will be hard to accept by some, but, though it is considered by many to be perfect in our day, linseed oil does possess some imperfect traits. This oil --so touted in our time as the best oil for oil painting-- exhibits two major drawbacks when used as a painting oil: First, it yellows badly and 'dries' on its surface much more quickly than beneath its surface. Second, as the outer layer dries it actually expands atop its still 'molten' core --leading to eventual wrinkling when used in thick and especially oil-rich paint-films. It doesn't take long, either. Often, this wrinkling can be seen within days of surface-drying ; and, once it wrinkles, it stays that way for good. And so, with oil painting, too much oil is a bad thing. [Too much pigment --and too much of anything-- is also a bad thing, but we're just talking oil here.]
The use of agents such as jelly mediums can easily dupe the painter and result in a too oil-rich paint film. There is a delusion engendered by working with a semi-solid thixotropic (gel-like) medium -- a delusion that must be guarded against. Because the jelly imparts a certain solidity to the paint, one feels safe where treachery can so easily prevail.
With a gel-type medium added to our paint, it all feels like its safely solid (not too runny). We don't see the hazard that our paint is oversaturated with oil. That's the effect of the gel. And anything that gels oil, even agents so widely used as aluminum stearate, will cause this delusion.
Of course, applications of linseed oil paint thicker than would normally be utilized with "regular" moderate or thin layers of oil-and-pigment painting will always produce more yellowing and wrinkling. But that is rather easy to guard against. Remember, due to the very fluid nature of simple raw oil, not much can be expediciously incorporated while painting, especially when dealing with vertical supports. What I mean is, too much oil in the mix and things will run with gravity. Because of this necessarily slight-use of oil, only very minor problems with yellowing and wrinkling will occur down the road.
But, repeating, any added thixotropic agent --such as a jelly medium --that allows excess oil to be utilized during painting will necessarily also allow additional yellowing and wrinkling. [Unless, of course, the fabled and non-yellowing "Holy Grail" substance for oil painting is discovered; such to allay the inherent defects of the Linseed oil.]
Again, this yellowing/wrinkling is due to the linseed oil constituent. Little else is involved in the 'crime' against the paint-film. As for yellowing, though other agents that have traditionally been combined with linseed oil --like resins-- may themselves slightly or faintly also yellow with age, the overwhelming culprit to visual yellowing in any varnish or paint....is linseed oil. These problems, yellowing and wrinkling, which are, again, inherent to this oil, cannot be totally negated in any way I am currently familiar with. [Washing, filtering, de-acidifying, and whatever else attempted or prescribed to rid the natural yellowing and wrinkling properties of LO ....will ultimately fail. So, again, we need the Holy Grail of oil paint and painting binders.]
Linseed oil and Megilp
The potential problem with megilp and other thixotropic mediums is this: all jelly mediums, when added to basic oil paint, fool the artist into thinking his/her paint is suitably thick and rich in pigment when it is often not truly so. The paint merely appears properly thick due to the thixotropic and up-standing character of the jelly medium. Of course, linseed oil does not exhibit its eventual yellowing property until its first-to-dry outer layer reaches a certain stage or degree of oxidation /drying. And so the artist has no way -- fresh appearance-wise -- to judge if his paint has been over-dosed with linseed oil by use of a thixotropic agent such as megilp or wax, or bentonite, or aluminum stearate, or aluminum hydrate. Nay, but that fiendish surprise will occur 'down the road'.
Again, it is linseed oil that causes the primary yellowing /wrinkling of oil paint. It simply turns yellow when that eventual drying state is reached. This inherent malady cannot be stopped, though I will inform others herein that there are a few agents, like wax or standoil, which initially appear to stop it ; but this is merely due to a slowing of the drying state. Eventually, the linseed film still "gets there"... then it yellows ...and, when applied in thick layers, it also browns and wrinkles.
BTW, drying agents -- like lead in many forms-- will speed the drying of linseed oil and allow it to reach its oncoming yellowing age-state sooner. Because of this, many authorities have falsely blamed drying agents for the yellowing of oil paint. Though I have found pure manganese driers can indeed chemically react and tarnish whites and other delicate tints, other driers -- such as lead-based types (and such combined with appropriate amounts of manganese agents)-- do not show in my tests as being causal to such browning. Instead, I have found the linseed to be the criminal instigator.
What I mean here is, without the drier, the linseed film coating the pigment particles will eventually dry anyway and reach the very same degree of yellowing. And so, with the incorporation of a drier, the paint film just reaches the yellowing state far quicker, leading many authorities to condemn driers out-right. [Near as I can tell, reading my history, this notion has been ongoing for a good three-and-a-half centuries. In addition to yellowing and wrinkling problems associated with use of lead as an oil paint and a drier, some painters of the 1600's felt it actually led to cracking. I cannot find any substantiation for this age-old condemnation. On the contrary, modern science seems to support the idea of lead actually aiding flexibility. Scrutiny of the old claims may likely reveal the problems to originate within the applied preliminary grounds. Typically, too thick an application of even a pure lead white priming can lead to such cracking. Then again, if thinly-applied, so as to not drown the weave of the canvas, even a glue chalk priming will fight cracking and remain quite lasting (only fill the weave ; do not go above it) ]
As noted earlier, the normal yellowing of properly-bound oil paint is not a major problem, as such paint is abundant with pigment and this pigment hides all but the outer-most layer of the existing oil binder (the coating, or pellicle). Normal daylight in a room will largely keep check against all but a slight yellowing (generally termed a "gallery tone"), which is always greatest in darkness, and especially prevalent to situations of damp dark.
In the case of jelly medium-use, what happens is the paint is too commonly allowed to be over-loaded with linseed oil and so, eventually, yellowing becomes even more visible due to less pigment-to-oil ratio in the films. Increasing the jelly-medium dosage allows too thick a film of linseed oil to be visually presented, compounding the visual effect of yellowing. [Again, the non-gel additives --like amber or copal mediums-- will, by the level of liquidity imparted to the paint mix, more easily show the artist the true nature of the compounded paint.]
The Yellowing Effect of Linseed Oil Diminishes Over Time (but not the wrinkling)
In time the usual and slight micro-coating of linseed oil sitting atop pigment will oxidize away leaving a rather pristine whiteness again. This doesn't take too long as solidified linseed oil --especially in thin coatings-- has no great toughness against oxidizing agents within the atmosphere. Many experts say linseed oil is tough to the elements. No. The linseed coating is truly rather weak. Just put a thin coat on a sheet of metal and expose it one day to the sun ; and it will noticeably oxidize. Though this oxidation occurs much more slowly indoors, still, it happens anyway. This is why, down through history, resins have been cooked into oil to make tough oil varnishes that can withstand atmospheric oxidizing (why enamel oil-based automobile paint withstands the elements better. Such paint or coatings resists oxidation due to a high resin-content -- and, by-the-way, very important,a UV-absorbing agent is most necessary to guard against sunlight-attack).
Historically, artists have found ways to increase resistence to oxidation of their paint by use of additives such as amber and copal varnish. Even a simple pine resin or pine oil varnish addition will increase the strength of a paint film and allay oxidation. Trouble is, indoors and away from direct Sunlight, the oil-film then becomes so resistant to oxidation that the yellowing (which is still predominantly the fault of the linseed oil) remains perfectly extant and thus visible for many -- many years longer than otherwise had the film not been strengthened by the addition of the varnish.
Thus, with linseed oil, anything added to strengthen the oil against oxidation ( such as a drop or two of amber varnish) will also extend the yellowing effect which is the nature of linseed oil. The good news is this yellowing is still slight and largely dependent on amounts of room lighting. [You see, the yellowing is "fugitive" when bombarded by certain scattering wavelengths of Sunlight -- even when these wavelengths are slight, as in a moderately-bright room.]
As to the relative strengths and merits of the various historically-known jelly mediums, the recent studies done by the Tate Gallery and the Canadian Conservation Institute show that a well-constructed megilp containing a proper balance and proportion of mastic resin to leaded drying oil is a decent medium when not used to excess. This is common sense after all and certainly explains why some great works known to be made with use of megilp have survived rather well through time. As to possible defects through over-use, it is my own opinion that any modern synthetic resin incorporated into a mechanical gelling agent (wax, bentonite, aluminum stearate, etc.) will suffer the very same yellowing, wrinkling problems associated with over-use of megilp. Again, this fault is due to the linseed oil ingredient and not the actual resin.
Also, be aware, chemically-produced jelling mediums applied pure ( that is, without pigment) and in thickness will eventually shrink and collapse as the oil oxidizes, and so become strongly wrinkled. As with over-thinned paint, avoid anything more than slightly-thick applications. Megilp is a chemical gellation and will eventually collapse as the oil and resin oxidize with age. In the case of the synthetic mechanical gels, the bulking substance/s do slump with age in the same degree as pigment does. Thus, no difference will be found between these mechanical formulations and regular megilp. The same ultimate condition (yellowing, wrinkling) happens to both.
Safe use of megilp/synthetic jelling mediums: glazing, scumbling, and conditions requiring slight thinning to allow brushing-ease come to mind as typical use for jelly mediums. If one wishes a megilp-carrier glaze, let it be done only as a thin application. To slather on even slightly-thick applications of any jelly medium is to invite the nasty linseed oil to create eventual havoc with the work.
There is one other slight effect we might consider. Mastic resin when used to create megilp remains its apparent true molecular self and is evenly scattered throughout the jelly film ( this info according to the micro-analysis of the Tate-CCI study). This means that with eventual micro-cracking of the paint film containing mastic resin, even weaker solvents utilized during future restoration may easily penetrate and attack the resin within the dried-out paint, essentially crumbling the whole film to a degree dependent on the amount of megilp used during original painting. Again, the painter should keep all uses on the slighter side, which is generally all that is really needed to glean proper effects. [ I do not consider this a great problem for most artists. Such slightly megilp-painted works have been successfully cleaned before. If megilp is another's method to masterful workmanship, I cannot fault the use. As I have noted earlier, great painters have fondly used megilp. I might easily propose the use of un-refined walnut oil instead of linseed oil as basis for jelly mediums. In normal room lighting, walnut oil shows little penchant for producing a gallery tone; nor does it wrinkle while drying in most --even thickly applied-- oil painting applications.]
I have no basis to believe the synthetic resins in use today will not also become eventually oxidized or otherwise soluble over time, or exhibit other break-down problems; not enough time has passed since their inception to make valuable predictions. I can say they appear to be quite strong and lasting, but I do not know them to be such.
James Groves, landscape painter.
November, 2000 Precautionary additions added July, 2002
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