Explainers · 2026-07-03 · ~3,900 words

Patreon for oil painting creators: linseed oil polymerization chemistry, pigment refractive index and opacity, siccative pigment drying rates, fat-over-lean at the polymer network level, varnish chemistry, and the Apple Tax

Oil painting Patreons retain when they deliver the chemistry layer that medium-selection notes and fat-over-lean rules cannot fully explain on their own: why linseed oil polymerizes rather than evaporates, how pigment refractive index determines whether a paint is transparent or opaque, which pigments accelerate drying by catalyzing the radical chain reaction, and what actually breaks at the polymer network level when fat-over-lean is violated. The oil painting audience spans YouTube, Instagram, and TikTok with varying iOS rates — the November 1, 2026 Apple Tax warrants action before October 31.

Linseed oil is not paint that dries — it polymerizes

The most consequential misunderstanding about oil paint is that it “dries.” Solvents like mineral spirits and turpentine evaporate, but the binder — the linseed oil or other drying oil — does not evaporate. It polymerizes: it reacts with atmospheric oxygen to form a crosslinked polymer network, a process called oxidative polymerization or autoxidation. A freshly painted oil painting weighs more when fully cured than when freshly applied, because it has incorporated oxygen from the atmosphere. The solvent fraction evaporates and the paint film loses volume and mass from that; the oil fraction gains oxygen and gains mass from that. The net change is small but the direction matters for understanding what is happening chemically.

Linseed oil is a mixture of triglycerides — glycerol molecules esterified with three fatty acid chains. The fatty acid composition of linseed oil is approximately 55% alpha-linolenic acid (18-carbon chain with three double bonds), 15% linoleic acid (18-carbon, two double bonds), 20% oleic acid (18-carbon, one double bond), and small amounts of saturated fatty acids with no double bonds. The double bonds are the reactive sites. Atmospheric oxygen attacks the allylic carbon-hydrogen bonds adjacent to the double bonds through a free-radical chain reaction mechanism: an initiating radical (produced by light, heat, or trace metal ions) abstracts a hydrogen atom adjacent to a double bond, producing a carbon-centered fatty acid radical. This radical reacts with oxygen to form a peroxy radical, which abstracts a hydrogen from another fatty acid chain, producing a new fatty acid radical and a fatty acid hydroperoxide. The chain continues until two radicals react with each other to form a crosslink between two fatty acid chains in separate triglyceride molecules. Over time, a three-dimensional crosslinked polymer network forms throughout the paint layer.

The yellowing mechanism arises from the same radical oxidation reactions. Alpha-linolenic acid has three double bonds arranged in a 1,4-pentadiene configuration. During oxidative polymerization, some of the intermediate oxidation products include conjugated enone and dienone groups — carbonyl groups conjugated with remaining double bonds that absorb light in the blue wavelength region (approximately 400–450nm). These chromophores accumulate in the film as oxidation proceeds, shifting the apparent color of the dried oil film toward yellow. The rate of chromophore formation scales with the polyunsaturated fatty acid content: alpha-linolenic acid with three double bonds produces the most chromophores per chain, linoleic acid with two produces fewer, oleic acid with one produces almost none.

This is the fundamental chemistry underlying the yellowing behavior differences between oils. Linseed oil, with approximately 55% linolenic acid, yellows the most rapidly. Walnut oil has approximately 15% linolenic acid and approximately 55% linoleic acid: its lower linolenic content produces fewer yellowing chromophores while its linoleic content still provides adequate two-bond crosslinking for a durable film. Walnut oil dries more slowly than linseed because linolenic acid with three double bonds initiates and propagates the radical chain more efficiently than linoleic acid with two — more reactive sites per chain means faster chain growth. The trade-off for medium selection documentation: linseed for warm areas where yellowing does not shift color perception significantly; walnut for whites, cool blues, and lavenders where a yellow shift of even 5–10% would visibly alter the color character of the area.

Stand oil, Liquin, and pre-formed polymer networks

Stand oil is linseed oil that has been heated to 270–330°C in the complete absence of oxygen for several hours. At these temperatures, Diels-Alder cycloaddition reactions occur between conjugated diene systems in adjacent fatty acid chains, forming direct carbon-to-carbon (C-C) bonds between chains through a pericyclic mechanism that does not require a radical intermediate and does not consume atmospheric oxygen. These pre-formed C-C crosslinks produce a partially polymerized starting material with dramatically higher viscosity than raw linseed — stand oil is typically three to ten times more viscous.

The chemical consequences of stand oil's pre-formed network differ from raw linseed in three measurable ways. First, leveling: the high viscosity creates a paint layer with high surface tension that flows and self-levels under gravity, reducing visible brushstroke texture in areas where a smooth film is desired. Second, reduced yellowing: the Diels-Alder reactions at high temperature consume many of the double bonds that would otherwise have been oxidized to form chromophores during atmospheric cure, so the fully cured stand oil film is clearer and yellows less than raw linseed. Third, very slow atmospheric drying: the remaining double bonds in stand oil are fewer and less reactive (many of the most reactive diene systems were consumed in Diels-Alder reactions), so the oxidative polymerization chain initiates and propagates more slowly once the medium is applied. Stand oil is appropriate in later fat layers where leveling and minimum yellowing are priorities and where the working pace of the painting allows for long inter-layer drying intervals.

Liquin is an alkyd medium — a synthetic polyester resin modified with unsaturated fatty acids, dissolved in mineral spirits. The polyester backbone is already partially crosslinked at the synthetic resin level, equivalent to a chemically pre-structured version of the Diels-Alder pre-polymerization in stand oil. The alkyd formulation also incorporates cobalt and manganese siccative additives at concentrations higher than occur naturally in plant oils. The combined effect is that when Liquin is applied with oil paint and exposed to atmospheric oxygen, the autooxidation radical chain initiates and propagates through the alkyd-incorporated fatty acids much faster than through a pure oil medium, reducing drying time to 24–48 hours versus the typical 3–7 days for linseed. The trade-off is that alkyd films eventually crosslink to a degree where they become difficult to remove or to varnish reversibly, which matters for conservation. For working painters, Liquin is documented at the layer level: which layers used Liquin and what drying time was observed before the next layer was applied.

Fat-over-lean at the polymer crosslink level

The fat-over-lean rule is usually stated as a working procedure — each layer must have a higher oil content than the layer beneath it. The reason is the differential in polymer network formation rate and mechanical properties between lean and fat layers. A lean layer (paint thinned with mineral spirits, or mixed 1:1 solvent to oil) produces a sparse crosslink network. The sparse network reaches maximum rigidity quickly because there are fewer potential crosslink sites and the sites that exist form bonds rapidly once solvent evaporates and the remaining oil is exposed to oxygen. The mechanical result is a layer that is rigid and has low elongation at break within a few days of application.

A fat layer (paint straight from the tube, or mixed with stand oil or linseed at a high oil-to-solvent ratio) produces a dense crosslink network. The dense network takes much longer to complete because more crosslink sites are available and the chain reaction must propagate through all of them. Critically, crosslink formation involves slight volume contraction: as polymer chains connect and tighten, the network occupies slightly less volume than the sum of its uncrosslinked components. This contraction continues to generate stress within the fat layer for weeks to months after the layer appears touch-dry at the surface.

The crack mechanism when lean is applied over fat: the lean layer bonds to the fat layer while the fat layer is still early in its crosslinking and contraction. As the fat layer continues to crosslink and contract over subsequent days and weeks, it exerts tensile stress on the bonded lean film above it. The lean film, having already reached maximum rigidity, cannot accommodate this stress through elastic deformation — it has low elongation and high stiffness. Stress accumulates at the interface and in the lean film until it exceeds the tensile strength of the lean film at some weakest point, and a crack initiates and propagates through the rigid lean layer. This is the paint cracking mechanism: not a failure of adhesion at the interface, but a tensile failure within the rigid lean layer caused by stresses generated in the still-contracting fat layer beneath it.

The long-term dimension: oil paint crosslinking continues for decades, not just days. The continued slow polymerization and contraction under old cured paintings generates stresses that accumulate over the life of the painting, eventually producing the craquelure patterns visible in antique oil paintings. The fat-over-lean structure delays and distributes these stresses by ensuring each layer is more flexible than the one beneath it — a flexible ground can deform slightly to absorb substrate movement and slow-polymerization contraction without transmitting full crack stress to the upper layers. For documentation, the critical variable that belongs in a Patreon paint note is the drying interval between layers in addition to the medium ratio per layer: the minimum drying interval before the next layer can safely be applied over a fat layer is specific to the oil content of the fat layer, the ambient temperature and humidity, and the pigment composition.

Pigment refractive index and the transparency-to-opacity spectrum

The refractive index is the ratio of the speed of light in a vacuum to the speed of light in a material. When light crosses a boundary between two materials with different refractive indices, some fraction of the light is reflected or scattered at the boundary rather than transmitted. The degree of scattering at each particle surface in a paint film determines whether the pigment is opaque (high scattering) or transparent (low scattering), and this is entirely determined by the refractive index difference (Δn) between the pigment particle and the surrounding oil medium. Linseed oil has a refractive index of approximately 1.48.

Titanium white in its rutile crystal form (TiO&sub2;) has a refractive index of 2.73 — the largest of any common artist's pigment. The Δn of 2.73 − 1.48 = 1.25 produces maximum scattering at every particle surface, meaning essentially all incident light is scattered back to the viewer rather than transmitted through the pigment layer. This is why titanium white has the highest hiding power of any pigment: at the same pigment loading, no other white achieves the same opacity. Lead white (basic lead carbonate, 2PbCO&sub3;·Pb(OH)&sub2;) has n ≈ 2.0, producing Δn ≈ 0.52 — still very high opacity, but less than titanium. Lead white also incorporates lead ions that are siccative (discussed in the next section), making it faster-drying as well as slightly more transparent, which historically made it preferred for underpainting where optical depth mattered and fast drying between layers was needed.

Cobalt blue (CoAl&sub2;O&sub4;, spinel structure) has n ≈ 1.74, producing Δn ≈ 0.26 relative to oil. This small difference means most incident light passes through the pigment particle rather than scattering at its surface, producing semi-transparency. When applied in thin layers over a white ground, cobalt blue allows light to pass through the glaze, reflect from the ground beneath, and return through the cobalt layer again — double-pass transmission through a selective blue absorber produces the luminous, deep blue that glazed cobalt achieves. This is why cobalt blue in thinly applied transparent layers looks qualitatively different from cobalt blue applied at full opacity: it is a different optical phenomenon, not merely a lighter value.

Phthalocyanine blue (PB15) achieves transparency through two separate mechanisms: a refractive index of approximately 1.5–1.7 (close to oil, producing low Δn) and very small primary particle size of 10–50nm versus titanium white at 200–300nm. Very small particles scatter less light regardless of refractive index because the scattering cross-section scales with particle size. In oil paint, phthalocyanine blue dispersed at fine particle size produces one of the most transparent blues available, able to produce intense, clear glazes at very small quantities. Its tinting strength is also enormous: phthalocyanine absorbs red-orange wavelengths with very high efficiency per unit mass, meaning that even tiny additions to a white base produce very strong color shifts. Documenting the exact volume ratio at which a mixing target is achieved is more important with phthalocyanine pigments than with most others, because the strong tinting strength means small errors in added quantity produce large color deviations.

Yellow ochre (iron oxyhydroxide, goethite, Fe·OOH) has n ≈ 2.0, producing moderate opacity. Its lightfastness is excellent because the iron oxide chromophore is chemically stable to UV exposure — the same mineral occurs in geological formations that have been light-exposed for geological time. Cadmium yellow (cadmium sulfide CdS for greenish-yellow, cadmium sulfoselenide for orange shades) has n ≈ 2.4–2.5, producing high opacity and brilliant color. Cadmium pigments have outstanding lightfastness because the sulfide chromophore absorbs light at a fixed UV threshold rather than through an easily bleached organic chromophore. Cadmium hue replacements use azo dyes or other organic pigments with lower lightfastness, which is the specific tradeoff being made when choosing student-grade cadmium hues over artist-grade cadmium.

Siccative pigments, radical scavengers, and the drying rate variation

Drying rate in oil paint is not uniform across pigments, even when the same medium is used at the same ratio. This is because certain pigments contain metal ions that catalyze or inhibit the oxidative polymerization radical chain, and others contain molecules that actively scavenge the radicals that initiate the chain.

Fast-drying pigments are those containing siccative metal ions, primarily cobalt, manganese, and lead. Transition metal ions catalyze oxidative polymerization by accepting and donating electrons: they react with fatty acid hydroperoxide intermediates to decompose them into alkoxy and hydroxy radicals, dramatically increasing the rate of radical generation and chain propagation. Cobalt blue and cerulean blue (both cobalt-containing) are the most potent siccatives in a standard artist palette — a layer mixed with cobalt blue may reach touch-dry in 24 hours where the same medium without cobalt would require three to five days. Raw umber contains manganese dioxide (MnO&sub2;), a classic desiccant in industrial coatings; a layer containing raw umber dries exceptionally fast regardless of the oil ratio used. This is the reason painters use raw umber in fast-drying ground layers: not just its neutral dark color but its chemical role in accelerating the polymerization of the entire paint layer.

Slow-drying pigments fall into two categories. Some provide no catalytic metal ions and thus offer only the baseline drying rate of the oil medium alone — cadmium pigments, for instance, are chemically inert in the oil film. Others actively inhibit drying by acting as radical scavengers: ivory black (lamp black from calcined bones, carbon black) contains graphitic carbon structures with delocalized electron systems that absorb and stabilize the radical intermediates that initiate and propagate the polymerization chain. Each radical that a carbon surface absorbs is a chain carrier that does not propagate further, reducing the overall rate of crosslink formation. Alizarin crimson (anthraquinone dye) has a high affinity for binding metal ions including cobalt and manganese, sequestering any siccative ions present in the medium and reducing their catalytic activity. A layer containing alizarin crimson will dry significantly more slowly than a layer with the same medium but a siccatively neutral pigment.

The practical documentation implication: in a multi-session painting, the pigment composition of each layer affects the actual drying time for that layer, not just the medium ratio. A cobalt-containing first layer at 1:1 solvent-to-oil may reach inter-layer safety faster than a pure-linseed layer of the same ratio without cobalt. Mixing cobalt blue with ivory black in the same layer produces a mixed drying rate that cannot be predicted from the medium ratio alone. The Patreon documentation note that records the pigment content of each layer alongside the medium ratio allows the creator to derive the drying time prediction from empirical data over multiple paintings, rather than from generic tables that do not account for pigment composition.

Two-phase drying: solvent evaporation and oil polymerization as separate processes

A freshly applied oil paint layer is simultaneously undergoing two distinct physical and chemical processes, which operate on very different timescales. Understanding them separately is what makes it possible to document inter-layer drying intervals accurately.

Phase 1 is solvent evaporation. Mineral spirits, OMS, and turpentine are volatile organics that evaporate from the paint surface over hours to a few days depending on the solvent type, the paint layer thickness, and ambient conditions. During this phase, the paint surface transitions from tacky to touch-dry. Touch-dry means the surface has formed a thin skin that does not leave residue on a fingertip with light contact — but the underlying paint is not cured. Immediately beneath a touch-dry surface, uncrosslinked oil still surrounds the pigment particles. In thick impasto layers, the interior oil may remain fully liquid for weeks after the surface appears completely dry. This is because oxygen diffusion through the already-forming surface skin limits the rate of oxidative polymerization in the interior: oxygen must diffuse from the air through the cured surface network to reach the still-reactive oil beneath, and the diffusion rate through a crosslinked polymer is much slower than through an open paint layer.

Phase 2 is oxidative polymerization. This begins at the paint surface the moment the wet paint layer is exposed to oxygen and continues for weeks to months to years, progressing inward from the surface as the crosslinked skin allows oxygen to diffuse into uncured material beneath. The crosslinked network does not stop developing when the painter can no longer detect surface tackiness — it continues to form new crosslinks and generate contraction stress throughout the life of the painting. The practical consequence for impasto painting is severe: a layer with significant thickness may have a cured, rigid surface over a liquid or semi-liquid interior for weeks. Applying a new layer over this state introduces an oil-rich liquid zone between two paint layers, and subsequent crosslinking of that zone generates delamination stress from below the new layer.

For documentation, the distinction matters because touch-dry is observable and documentable in real time (the creator knows when a layer passes the fingertip test), but full cure (phase 2 completion) is not directly observable and extends over a much longer period. The inter-layer interval documented in a Patreon paint note is the interval until touch-dry plus whatever empirically-derived additional waiting time the creator has established for layers of that oil content and thickness. The creator who has documented drying intervals across ten paintings knows, from that accumulated data, how long a specific type of fat layer needs before a subsequent layer is safe — that empirical knowledge is the Patreon exclusive, not the formula.

Varnish chemistry: damar, acrylic, and the isolation coat

Varnish is applied over a fully cured oil painting for two reasons: to equalize surface sheen (oil paint cures to variable gloss levels depending on oil content and pigment absorption) and to provide a removable protective layer for conservation. The chemical nature of the varnish determines whether it will remain removable without damaging the paint beneath, and how it ages optically.

Damar resin is a diterpene resin from tropical Shorea and Balanocarpus trees, dissolved in turpentine or mineral spirits at concentrations of 20–30% by weight. Applied as a solution, the solvent evaporates and the resin deposits as a clear, hard film. Damar is not crosslinked — unlike oil paint, damar film does not form a polymer network; the diterpene molecules remain as a high-molecular-weight solution that simply deposits when the solvent evaporates. This is why damar is soluble in mineral spirits and turpentine: it was deposited from solution and can be redissolved. Over time, damar yellows as the diterpene molecules undergo slow oxidation, forming chromophores similar to those in yellowed linseed oil. Studies show that damar varnish can add a perceptible yellow cast within five to ten years, and may accumulate significant yellowing over decades. Conservation-grade damar application requires working in a dust-free environment (the non-crosslinked surface is sticky when wet and takes longer to firm up than crosslinked films) and brushing in one direction only to avoid streaks.

Acrylic varnishes (commercially as Gamvar, Golden MSA, Liquitex High Gloss Varnish) use synthetic acrylic polymers or mineral spirit acrylic (MSA) resins that form stable films with minimal yellowing because the acrylic polymer backbone contains no unsaturated double bonds that could form chromophores through oxidation. MSA varnish is removable with mineral spirits; water-based acrylic varnishes are removable with dilute ammonia solution. These varnishes are the current conservation-recommended choice for contemporary oil paintings because of their minimal yellowing, stable optical properties, and reversibility. The trade-off is surface character: some painters find the surface sheen of acrylic varnish subtly different from traditional damar, with a slightly different micro-texture and light response.

The isolation coat is a thin, non-removable layer of acrylic polymer medium (PVA emulsion or gloss medium) brushed thinly over the fully cured oil paint surface and allowed to dry completely before any varnish is applied. Its function is to seal the somewhat porous, reactive surface of a cured oil painting and prevent the subsequently applied varnish from penetrating into the paint layer. A fully cured oil paint surface is not inert: it contains free hydroxyl and carboxyl groups from the residual oxidation products of the polyunsaturated fatty acids, and these groups can form ester or hydrogen bonds with some varnish molecules that enter the paint layer and become effectively non-removable. The isolation coat provides a stable, sealed acrylic surface that the varnish bonds to without penetrating, keeping the varnish in a removable top layer. The isolation coat must be fully cured (minimum 48 hours at room temperature) before varnish application. It is non-removable by design — it stays on the painting permanently as part of the paint surface structure.

Color mixing at the pigment dispersion level

Color mixing in oil paint is subtractive: each pigment absorbs certain wavelengths from incident white light and reflects or transmits the rest. Mixing two pigments combines their subtractive filter effects — the mixture absorbs everything that either pigment absorbs separately, reflecting only what both pigments fail to absorb. This is why mixing more than two or three pigments tends to produce increasingly neutral or dark colors: the combined absorption spectrum of multiple pigments extends across more and more of the visible range, leaving less light reflected.

Tinting strength is the most consequential pigment property for mixing documentation. Tinting strength is the amount of color change per unit weight of pigment added to a white base. Phthalocyanine blue has the highest tinting strength of any common blue: each milligram of phthalocyanine absorbs red-orange wavelengths with very high efficiency, and adding a small quantity to a white-dominated mix produces a very strong color shift. A painter who adds phthalocyanine blue by “feel” rather than by measured ratio will find that additions that appear correct at first overpower the mixture within a few brushstrokes, producing an extreme blue bias. The same behavior applies to phthalocyanine green, quinacridone magenta, and Hansa yellow: all have very high tinting strength relative to their weight, requiring precise small-quantity control for predictable mixing. Documenting the mixing ratio as approximate percentages by volume — “approximately 2% phthalocyanine blue, 15% titanium white, 83% cerulean blue” — is what makes the color note reproducible to a patron at a different color balance in their tube.

The limit of pigment mixing at the physics level: any mixture of more than three spectrally distinct pigments absorbs wavelengths across a broad fraction of the visible spectrum. The broadening of absorption with each added pigment progressively reduces the saturation of the reflected light, pushing the mixture toward gray or brown. Experienced painters restrict most mixing to two primary pigments plus titanium white for tints, using a third pigment only for narrow adjustments. Documenting this approach explicitly — “this green was mixed from two pigments only, then desaturated with a small addition of the complementary” — teaches the patron the structural reason that complex palettes produce muddy results, which is a more durable piece of knowledge than any specific color formula.

Apple Tax for oil painting creator audiences

Oil painting creators have moderate-to-strong iOS exposure that varies meaningfully by platform and content type. YouTube oil painting tutorials and time-lapse content: 55–65% iOS — longer tutorial content is more commonly watched on desktop or connected television in a studio context, producing a smaller iOS share than content categories where the primary discovery and consumption mechanism is mobile feed browsing. Instagram oil painting finished-work photography and process reels: 65–75% iOS — Instagram is predominantly a mobile platform consumed through the app, and oil painting photographs and short process clips are primarily discovered in the feed and Explore page. TikTok alla prima and plein air process videos: 70–80% iOS — short-form process content performs well on TikTok and is discovered almost entirely on mobile.

In dollar terms beginning November 1, 2026: at $350/month with 60% iOS, approximately $63/month ($756/year) in Apple fees. At $500/month with 65% iOS, approximately $97.50/month ($1,170/year). At $300/month with 75% iOS (TikTok-primary), approximately $67.50/month ($810/year). Enable Patreon’s web-only billing toggle in Creator Settings before October 31, 2026. Update YouTube channel descriptions, Instagram bio links, and TikTok profile links to point directly to the Patreon web URL. For YouTube-primary creators, verify that the channel description link opens a web browser to the Patreon URL rather than launching the Patreon iOS app for viewers who have the app installed — the link must route to the web URL, not to app content. Test the full subscription flow from an iPhone browser before November 1 to confirm no Apple IAP dialog appears.

KeepTier is a self-hosted membership page for creators who want 100% of their tier revenue and zero Apple tax. Plans start at $9/month.