Why “Medium” Is So Often Confusing
Few terms in painting are used with greater broad-stroke ambiguity than “medium“. In common art-related conversations, some may use the word to refer to the binder in a paint system, a liquid additive mixed into a paint system, a solvent-rich thinning mixture, or even the entire material category in which an artwork is made. An artist may say “oil is the medium,” “add some medium,” or “the painting is done in mixed media,” and each statement uses the same word to communicate very different, but sometimes related, concepts.
This confusion is not merely sloppy usage. It reflects the fact that “medium” has several legitimate meanings that developed in different traditions: general art discourse, workshop practice, and technical descriptions of paint composition. The result is a term that remains useful, but only if its meaning is kept context-specific. Clarifying the term matters because misunderstandings about medium often lead to misunderstandings about binding strength, solvent use, fat-over-lean, drying, and long-term film stability. It is therefore worth distinguishing the historical breadth of the term from its stricter material meanings.
Some Background for the Curious…
In English, the term “medium” is attested from the late sixteenth century in the sense of a middle ground, middle state, or intervening position. It derives from the Latin medium, meaning “the middle,” “midst,” “center,” or “interval,” from the neuter form of medius, meaning in the middle or between. More distantly, it is related to the Indo-European root medhyo-, also meaning middle.
This origin is more than a linguistic curiosity. It helps explain why the word later developed such a wide range of meanings. From the idea of something situated between two points came the secondary sense of an intervening substance through which a force, action, or quality is conveyed. Closely related to this was the sense of an intermediate agency or channel of communication. Both developments are crucial to the term’s later history in art.
When applied to artistic practice, the term medium came to designate the material means by which an image, form, or idea is brought into visible existence. In this broad sense, a medium is not merely a substance but a means of realization: that through which artistic intention becomes materially present. Thus, oil, watercolor, ink, pastel, encaustic, and tempera may each be described as artistic media. Here, the word refers not to a single ingredient within the paint, but to the overall material system or technical mode in which a work is made.
This broad usage remains standard in art history, museum cataloging, and studio pedagogy. Descriptions such as oil on canvas, watercolor on paper, or mixed media all rely on this categorical sense. In such contexts, “medium” refers to the artistic material or method as a whole, not to a bottled additive mixed into paint during use.
The later history of the word also helps explain some modern ambiguity. Because medium could mean both an intervening substance and a channel of conveyance, it eventually expanded into other domains, including communication and print culture, and later into the modern plural form media. In painting, however, the older material sense remains especially important: a medium is, fundamentally, the means by which form is carried into matter.
It is in the context of oil painting, however, that the term “medium” often causes the most confusion. In light of the word’s etymology, it is easy to see how medium came to suggest an intervening material within the paint system; that is, something understood as standing between pigment, binder, and application. Yet this broad conceptual sense should not be confused with stricter technical definitions. In studio practice, added media may alter consistency, flow, gloss, transparency, and drying behavior, but not all are identical in function. Some increase the amount of film-forming material in the paint, while others act chiefly as temporary thinning agents or handling modifiers. For that reason, medium in painting is often best treated as a contextual studio term rather than as a single precise technical category.
At the same time, that analogy should not be pressed too far. Describing an oil painting medium as the thing “between the raw pigment and the final dried film” can be misleading, especially when one is referring to additive media used in studio practice. Such phrasing risks collapsing an important distinction between pigment and paint. Pigment, strictly speaking, is the particulate colorant; it does not become part of a usable paint until it is dispersed in a binder or vehicle. In technical terms, it is the binder or vehicle that first mediates between loose pigment and the eventual dried film by carrying, surrounding, and ultimately fixing the pigment into a coherent layer. (Bentley, 2001; Gettens & Stout, 1966) By contrast, many substances called “mediums” in studio practice are not the original constitutive vehicle of the paint but later additions to an already existing paint system. Such added media may increase binder content, alter rheology, affect gloss or transparency, or modify drying behavior, but they do not all occupy the same structural role within the paint. (Gooch, 2002; Izzo et al., 2014) In that sense, the term “medium” in oil painting can obscure an important distinction between what makes paint paint and what merely modifies paint once it is made.
The Vehicle or Binding System
If the previous discussion shows why the term medium became so broad in artistic and studio usage, technical paint literature narrows the term considerably. In this more specific sense, medium often refers to the vehicle or binder: the liquid or semi-liquid portion of the paint that carries the pigment and ultimately forms the binding film. In oil paint, that binder is typically a drying oil, most commonly linseed oil, though walnut and safflower oils are also used in certain formulations.
Under this stricter definition, the essential components of paint are, at a minimum, pigment and binder (or vehicle). The binder wets and disperses the pigment, enabling the paint to be applied, and then forms a coherent solid film as it dries or cures. In oil painting, this process occurs not primarily through simple evaporation, but through oxidation and polymerization of the oil.
This narrower use appears in technical literature and among authors such as Ralph Mayer, who often describe paint as pigment plus vehicle, with other materials treated as optional additions or modifiers. In that sense, the binder itself may be regarded as the paint’s fundamental medium, at least in the structural or film-forming sense.
A common source of confusion, however, is the widespread studio practice of adding plain drying oil (such as linseed, stand, walnut, or safflower oil) to paint and referring to it simply as a medium. In one sense, this is entirely understandable, since the painter is adding more of the same general class of material that already serves as the paint’s binder or vehicle. At the same time, this practice shows why the term can remain unstable: the added oil is not merely an external modifier, but an increase in the film-forming component of the paint itself.
Such an addition is not materially neutral. Commercial oil colors are generally formulated around the oil demand, handling properties, and drying behavior of particular pigments, which can vary substantially from one color to another. Adding extra drying oil may increase flow, gloss, and transparency, but it can also shift the binder–pigment balance of the paint, alter drying behavior, and change the mechanical character of the resulting film. More oil should therefore not be assumed to make paint automatically safer or more durable simply because it is fatter. Depending on the pigment, layer structure, and amount used, excessive or indiscriminate oil addition may disrupt the stock formulation’s balance rather than improve it. (Bentley, 2001; Gettens & Stout, 1966; Gooch, 2002; Izzo et al., 2014)
In this stricter technical sense, then, medium refers most properly to the binder or vehicle that makes the paint a paint in the first place. In ordinary studio usage, however, the word more often refers to a material added to an already constituted paint in order to modify handling, surface quality, or drying behavior. The overlap between these two meanings is one reason the term remains useful yet persistently ambiguous.
Additive Media
In everyday studio use, the term medium most often refers to a material added to an existing paint system (pigment plus vehicle) to alter its handling, appearance, or drying behavior. This is probably the most common contemporary meaning of the term among painters. In this practical sense, a medium may increase flow, improve leveling, increase gloss, deepen transparency, alter body, modify tack, or affect the rate and character of drying. Examples include added linseed oil (a drying oil that increases flow, gloss, and binder content), stand oil (a heat-bodied linseed oil that increases smoothness, leveling, and enamel-like flow), alkyd media (oil-modified synthetic resin media that improve flow and often accelerate drying), wax media (wax-containing additions that alter body, drag, texture, and often reduce gloss), oleoresinous mixtures (oil–resin combinations that increase gloss, body, and film-forming character), and a range of commercial proprietary mediums (manufacturer-formulated blends designed to modify handling, drying, or surface effects). In studio usage, this category is sometimes extended more loosely to include solvents or solvent-rich mixtures, since these also affect viscosity, flow, and application, even though their volatile component does not remain in the final paint film in the same way. These do not “create” the paint from scratch; rather, they are introduced into an already constituted paint system in order to modify its working properties. For that reason, they are more accurately understood as additives to the paint system, even though studio practice commonly labels them simply as “mediums.”
This everyday usage is practical and deeply established, but it groups together materials that do not all function in the same way. Some studio media increase the amount of binder or other film-forming material in the paint layer. Oil additions such as linseed or stand oil, for example, enrich the paint with additional drying oil and tend to increase flow, gloss, and transparency while also affecting drying behavior. Alkyd media likewise contribute film-forming material, but they do so through a different resin chemistry and are often used to accelerate drying while improving flow and leveling. Wax-containing media, by contrast, can alter body, drag, matteness, and texture, often reducing gloss and changing the tactile response of the paint. Other mixtures may have a substantial solvent content and function primarily by temporarily reducing viscosity during application, leaving relatively little of that volatile portion behind after evaporation.
For that reason, the studio category medium is not a single functional class so much as a practical umbrella term. What these materials share is not one precise chemical role, but the fact that painters use them to adjust how paint behaves. One medium may chiefly enrich the paint with more binder; another may chiefly alter rheology; another may mainly change application and brushing feel; another may modify drying rate, surface quality, or translucency. Their grouping under one name reflects workshop convenience more than material uniformity.
One further source of confusion is that some studio media blur the line between modifier and constituent (even though a material can be both). Here, a modifier means a material added chiefly to change the paint’s handling, flow, drying, or surface qualities, while a constituent means a material that remains as part of the final dried paint film. When a painter adds linseed oil or stand oil to paint, the added material is not merely an external aid to application; it becomes part of the binding film itself. By contrast, when a painter uses a solvent-heavy mixture, much of its effect may be temporary, since the volatile portion evaporates rather than remaining as part of the final film. Studio language often calls both of these things “medium,” even though, materially, they occupy quite different roles.
So when painters use the word medium colloquially, they usually mean a paint-modifying additive, not the binder in the strict chemical sense and not the medium in the broader art-historical sense. The studio meaning is therefore legitimate and useful, but it is broader, more elastic, and less precise than the terminology of paint chemistry.
Solvent: A Distinct Material Category
A solvent, while certainly capable of modifying paint behavior, is not usually regarded as a medium in the structural sense developed above. A solvent is a liquid capable of dissolving or swelling (causing it to soften and expand) another material. In oil painting, common examples include gum spirits of turpentine and mineral spirits.
Their roles are primarily temporary. Solvents may reduce viscosity, aid brushing, dissolve natural resins, or assist in cleaning tools. What they do not ordinarily do is form the permanent binding film of the paint layer. Because they evaporate, they do not function as the durable adhesive matrix that binds pigment particles into a coherent, lasting film on the support.
For this reason, it is technically misleading to treat solvents as equivalent to media, even though some artists do so in casual studio speech. A solvent may be included as one component of a painting medium sold for studio use, but the solvent itself is not the binding medium of the dried paint film.
This distinction matters in practice because painters sometimes assume that “more fluid” means “more medium.” In fact, a more fluid paint may simply contain more solvent and therefore less binder per given volume of wet paint. In modest amounts, this may merely change handling, but in excess, it can contribute to films that are relatively lean, underbound, or mechanically weaker than intended. (Gooch, 2002; Pearce, 2019)
Diluent: A Functional Rather Than Strictly Material Term
The term diluent is sometimes used where artists might otherwise say solvent, though the two words are not identical in emphasis. A solvent is generally defined by its ability to dissolve or swell another material, whereas a diluent is defined more by its practical function of reducing concentration, body, or viscosity. In this sense, solvent emphasizes chemical action, while diluent emphasizes a thinning function.
In oil painting, mineral spirits are often used as diluents because they reduce thickness and improve spread without contributing to the permanent paint film. A given liquid may therefore serve as both a solvent and a diluent, depending on the context. Because usage in art literature is not fully consistent, the safest generalization is that both terms usually refer to temporary modifiers, whereas the binder or other true film-forming medium remains part of the cured paint structure.
Driers and Siccatives
Another category often confused with mediums is driers, also called siccatives. These are materials, often metal-containing compounds, that accelerate the oxidation and curing of drying oils. Historically, lead and manganese compounds were common; modern products more often rely on cobalt, zirconium, calcium, or manganese systems.
Driers are not mediums in the general structural sense developed here. Although they affect the behavior of the paint system, they do so not by acting as the binder or by substantially increasing the film-forming portion of the paint, but by catalyzing the chemical reactions that drive the drying oil’s curing. In other words, they influence the rate and character of drying without themselves serving as the principal material that carries pigment and forms the binding film.
For that reason, driers are better understood as reactive additives than as media proper. They are normally used in small amounts to influence curing, not to alter the paint’s body in the same way as oils, waxes, or resinous media, and not to function as a temporary thinning agent in the way solvents do. Their role is therefore distinct from both the binder and the more common studio meaning of medium as a handling additive.
Because they alter chemical drying rather than merely viscosity or flow, driers must be used cautiously. Excessive use can contribute to embrittlement, wrinkling, surface defects, or poor long-term stability.
At the same time, historical practice complicates the terminology. Some traditional painting mediums combined oil, resin, solvent, and siccative in a single mixture, which helped blur the boundaries between binder, additive medium, solvent, and drier. That history helps explain why artists sometimes classify all liquid additions under the umbrella term “medium,” even when the materials involved serve very different functions within the paint system.
Mayer, Gottsegen, and the Value of Distinction
Ralph Mayer and Mark David Gottsegen are especially useful in this discussion because both help separate workshop language from more exact material description, though they do so in somewhat different ways.
Mayer’s framework tends to emphasize the structural composition of paint as pigment plus vehicle, with resins, waxes, driers, and other substances treated as additions serving particular purposes. His language often directs the reader toward a more material and process-based understanding of paint, one in which the vehicle or binder occupies a privileged place because it is the component that carries the pigment and forms the eventual film. In this respect, Mayer is especially valuable for clarifying the difference between what constitutes a paint and what is only later added to modify it.
Gottsegen, by contrast, writes with somewhat greater attention to the breadth and variability of contemporary art-materials terminology. He acknowledges the broad, practical way artists use the word medium, while also classifying many such materials more specifically as additives, varnishes, extenders, or modifiers, depending on their actual role. His approach is especially helpful because it preserves the legitimacy of studio use without sacrificing the need for functional distinction.
Taken together, these authors support a practical conclusion: the word medium is not wrong, but it is often too broad to be sufficiently exact on its own. Good instruction therefore benefits from naming the actual function of the material in question, whether it is acting as a binder, vehicle, additive medium, solvent, diluent, or drier, rather than relying on the single word medium to do all the explanatory work.
Why the Distinction Matters in Practice
While I am often accused of hyper-hair-splitting, these distinctions are not merely semantic. They affect both how painters work and how paintings age. When artists over-thin paint with solvent, they may mistakenly believe they are adding “medium,” when in fact they are reducing the proportion of binder in the wet mixture and risking paint films that are relatively lean, underbound, or mechanically weaker than intended. (Pearce, 2019; Sutherland, 2001) When they misunderstand fat over lean, they may focus on how fluid a passage looks rather than on how much film-forming material it actually contains. And when they assume that solvent “speeds drying,” they confuse two different processes: evaporation, by which volatile liquids leave the paint, and oxidative curing, by which drying oils polymerize into a solid film. (Dawson, 2007)
The result is not only conceptual confusion but practical error. A paint layer may appear easy to brush because it contains more solvent, yet still be relatively poor in binder. A layer may feel dry to the touch because solvent has evaporated from the surface, while the oil beneath continues curing much more slowly. Likewise, a passage made richer with oil may be “fatter” in the material sense even if it is not especially fluid, whereas a passage made more fluid with solvent may actually be leaner. Terminology matters because visual or tactile impressions do not always reveal what is happening structurally within the paint film.
Precise language, therefore, improves practical judgment. A painter who understands whether a material is functioning as a binder, solvent, diluent, drier, or additive medium is in a much better position to control handling, surface quality, drying behavior, and long-term durability. The point is not to police vocabulary for its own sake, but to make material decisions with greater clarity about what each addition is actually doing in the paint system.
A Practical Working Definition
Based on what we have covered thus far, a viable and useful definition would be:
Medium: a context-dependent term in painting. It may refer broadly to the artistic material system, more narrowly to the binder or vehicle of paint, or in common studio usage to a material added to paint to modify handling, appearance, drying, or film behavior. In this studio sense, some media become constituents of the final paint film, while others function chiefly as temporary modifiers. For clarity, the term should therefore be specified by role whenever precision matters: binder, vehicle, additive medium, solvent, diluent, or drier.
In Summary
The confusion surrounding the term medium in painting is not accidental. Historically, the word has carried the broad sense of a means, vehicle, or intervening substance, and this semantic flexibility has allowed it to develop several legitimate but distinct meanings across art history, studio practice, and technical descriptions of paint. In one context, medium names the material system of a work; in another, it refers to the binder or vehicle of paint; in another still, it serves as a practical studio term for an additive used to modify handling, appearance, drying, or film behavior.
The problem, then, is not that the word is wrong, but that it is often too broad to be sufficiently exact on its own. Some studio media materially contribute to the final paint film, while others act chiefly as temporary or auxiliary modifiers. Solvents, diluents, and driers may all affect the behavior of a paint system, but they do not therefore function as media in the same structural sense. For that reason, the most useful approach is not to abandon the term, but to specify the role being discussed whenever precision matters.
Doing so improves both instruction and practice. It helps painters distinguish between what constitutes the paint and what merely modifies it, between what remains in the film and what only assists application, and between what alters handling temporarily and what changes the long-term structure of the paint layer. Clear terminology does not simply tidy up language; it supports better material judgment, more predictable working methods, and greater long-term stability in the finished work.
Resources
Mayer, R. (1991). The Artist’s Handbook of Materials and Techniques (5th ed.). New York: Viking.
Gottsegen, M. D. (2006). The Painter’s Handbook: A Complete Reference (2nd ed.). New York: Watson-Guptill.
Carlyle, L. (2001). The Artist’s Assistant: Oil Painting Instruction Manuals and Handbooks in Britain 1800–1900. London: Archetype.
Eastlake, C. L. (1847). Materials for a History of Oil Painting. London: Longman, Brown, Green, and Longmans.
Gettens, R. J., & Stout, G. L. (1966). Painting Materials: A Short Encyclopaedia. New York: Dover.
Doerner, M. (1949). The Materials of the Artist and Their Use in Painting. New York: Harcourt, Brace.
Kühn, H. (1986). “Binding Media, Varnishes and Adhesives.” In A. Roy (Ed.), Artists’ Pigments: A Handbook of Their History and Characteristics, Vol. 1. Washington, DC: National Gallery of Art.
Ball, P. (2001). Bright Earth: Art and the Invention of Color. London: Penguin.
Wadum, J. (1998). “Historical Overview of Panel-Making Techniques in the Northern Countries.” In Looking Through Paintings. London: Archetype.
Mills, J. S., & White, R. (1994). The Organic Chemistry of Museum Objects (2nd ed.). Oxford: Butterworth-Heinemann.
