Lightfastness is a property of a

colourant A colourant/colour additive (British spelling) or colorant/color additive (American spelling) is a substance that is added or applied in order to change the colour of a material or surface. Colourants can be used for many purposes including printing ...

such as

dye A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution and ...

pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compou ...

that describes its resistance to fading when exposed to light. Dyes and pigments are used for example for

dyeing Dyeing is the application of dyes or pigments on textile materials such as fibers, yarns, and fabrics with the goal of achieving color with desired color fastness. Dyeing is normally done in a special solution containing dyes and particular c ...

fabric Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, different fabric types, etc. At first, the word "textiles" only referred to woven fabrics. However, weaving is not ...

plastic Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adapta ...

s or other materials and manufacturing

paint Paint is any pigmented liquid, liquefiable, or solid mastic composition that, after application to a substrate in a thin layer, converts to a solid film. It is most commonly used to protect, color, or provide texture. Paint can be made in many ...

s or printing inks. The bleaching of the color is caused by the impact of

ultraviolet radiation Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...

in the chemical structure of the

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and b ...

s giving the color of the subject. The part of a molecule responsible for its color is called the

chromophore A chromophore is the part of a molecule responsible for its color. The color that is seen by our eyes is the one not absorbed by the reflecting object within a certain wavelength spectrum of visible light. The chromophore is a region in the mo ...

. Light encountering a painted surface can either alter or break the

chemical bond A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules and crystals. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds, or through the sharing of ...

s of the pigment, causing the colors to

bleach Bleach is the generic name for any chemical product that is used industrially or domestically to remove color (whitening) from a fabric or fiber or to clean or to remove stains in a process called bleaching. It often refers specifically, to ...

or change in a process known as

photodegradation Photodegradation is the alteration of materials by light. Commonly, the term is used loosely to refer to the combined action of sunlight and air, which cause oxidation and hydrolysis. Often photodegradation is intentionally avoided, since it destro ...

. Materials that resist this effect are said to be lightfast. The

electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging fro ...

of the sun contains

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...

s from gamma waves to radio waves. The high energy of

in particular accelerates the fading of the dye. The

photon energy Photon energy is the energy carried by a single photon. The amount of energy is directly proportional to the photon's electromagnetic frequency and thus, equivalently, is inversely proportional to the wavelength. The higher the photon's frequency, ...

of UVA-radiation which is not absorbed by atmospheric

ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the l ...

exceeds the dissociation energy of the carbon-carbon

single bond In chemistry, a single bond is a chemical bond between two atoms involving two valence electrons. That is, the atoms share one pair of electrons where the bond forms. Therefore, a single bond is a type of covalent bond. When shared, each of ...

, resulting in the cleavage of the bond and fading of the color.

Inorganic In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemist ...

colourants are considered to be more lightfast than

organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ Chemistry * Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...

colourants. Black colourants are usually considered the most lightfast. Lightfastness is measured by exposing a sample to a

lightsource Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 ...

for a predefined period of time and then comparing it to an unexposed sample.

Chemical processes

During the fading, colourant molecules undergo various chemical processes which result in fading. When a UV-photon reacts with a molecule acting as colourant, the molecule is excited from the

ground state The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state. ...

to an excited state. The excited molecule is highly reactive and unstable. During the

quenching In materials science, quenching is the rapid cooling of a workpiece in water, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as ...

of the molecule from excited state to ground state, atmospheric

triplet oxygen Triplet oxygen, 3O2, refers to the ''S'' = 1 electronic ground state of molecular oxygen (dioxygen). It is the most stable and common allotrope of oxygen. Molecules of triplet oxygen contain two unpaired electrons, making triplet oxygen an unus ...

reacts with the colourant molecule to form

singlet oxygen Singlet oxygen, systematically named dioxygen(singlet) and dioxidene, is a gaseous inorganic chemical with the formula O=O (also written as or ), which is in a quantum state where all electrons are spin paired. It is kinetically unstable at ambi ...

and

superoxide In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of t ...

oxygen radical. The oxygen atom and the superoxide radical resulting from the reaction are both highly reactive and capable of destroying the colourants.

Photolysis

Photolysis Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...

, ''i.e.,'' photochemical decomposition is a chemical reaction where the compound is broken down by the photons. This decomposition occurs when a photon of sufficient energy encounters a colorant molecule bond with a suitable dissociation energy. The reaction causes homolytic cleavage in the chromophoric system resulting in the fading of the colourant.

Photo-oxidation

Photo-oxidation, ''i.e.,'' photochemical

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...

. A colorant molecule, when excited by a photon of sufficient energy, undergoes an oxidation process. In the process the chromophoric system of the colorant molecule reacts with the atmospheric oxygen to form a non-chromophoric system, resulting in fading. Colorants which contain a

carbonyl group In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containi ...

as the chromophore are particularly vulnerable to oxidation.

Photoreduction

Photo-reduction, ''i.e.,'' photochemical reduction. A colorant molecule with an unsaturated

double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betwee ...

(typical to

alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic ...

s) or

triple bond A triple bond in chemistry is a chemical bond between two atoms involving six bonding electrons instead of the usual two in a covalent single bond. Triple bonds are stronger than the equivalent single bonds or double bonds, with a bond order o ...

(typical to

alkyne \ce \ce Acetylene \ce \ce \ce Propyne \ce \ce \ce \ce 1-Butyne In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...

s) acting as a chromophore undergoes reduction in the presence of hydrogen and photons of sufficient energy, forming a saturated chromophoric system. Saturation reduces the length of the chromophoric system, resulting in the fading of the colorant.

Photosensitization

Photosensitization Photosensitizers produce a physicochemical change in a neighboring molecule by either donating an electron to the substrate or by abstracting a hydrogen atom from the substrate. At the end of this process, the photosensitizer eventually returns to ...

, ''i.e.,'' photochemical sensitization. Exposing dyed cellulosic material, such as plant-based fibers, to sunlight allows dyes to remove hydrogen from the cellulose, resulting in photoreduction on the cellulosic substrate. Simultaneously, the colorant will undergo oxidation in the presence of the atmospheric oxygen, resulting in photo-oxidation of the colourant. These processes result in both fading of the colorant and strength loss of the substrate.

Phototendering

Phototendering, ''i.e.,'' photochemical tendering. As a result of UV light, the substrate material supplies hydrogen to the colourant molecules, reducing the colorant molecule. As the hydrogen is removed, the material undergoes oxidation.

Standards and measure scales

Some organizations publish standards for rating the lightfastness of pigments and materials. Testing is typically done by controlled exposure to

sunlight Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when ...

, or to artificial light generated by a

xenon arc lamp A xenon arc lamp is a highly specialized type of gas discharge lamp, an electric light that produces light by passing electricity through ionized xenon gas at high pressure. It produces a bright white light to simulate sunlight, with applications ...

Watercolors Watercolor (American English) or watercolour (British English; see spelling differences), also ''aquarelle'' (; from Italian diminutive of Latin ''aqua'' "water"), is a painting method”Watercolor may be as old as art itself, going back to t ...

, inks,

pastel A pastel () is an art medium in a variety of forms including a stick, a square a pebble or a pan of color; though other forms are possible; they consist of powdered pigment and a binder. The pigments used in pastels are similar to those use ...

s, and colored pencils are particularly susceptible to fading over time, so choosing lightfast pigments is especially important in these media. The most well known scales measuring the lightfastness are the Blue Wool Scale, Grey scale and the scale defined by ASTM (American Standard Test Measure). On the Blue Wool Scale the lightfastness is rated between 1–8. 1 being very poor and 8 being excellent lightfastness. In grey scale the lightfastness is rated between 1–5. 1 being very poor and 5 being excellent lightfastness. On ASTM scale the lightfastness is rated between I-V. I is excellent lightfastness and it corresponds to ratings 7–8 on Blue Wool Scale. V is very poor lightfastness and it corresponds to Blue Wool scale rating 1. The actual lightfastness is dependent on the strength of the radiation of the sun, so lightfastness is relative to geographic location,

season A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and ...

, and exposure direction. The following table is listing suggestive relations of the lightfastness ratings on different measure scales and the relation relative to time in direct sunlight and normal conditions of display: away from a window, under indirect sunlight and properly framed behind a UV protective glass.

Test procedure

The relative amount of fading can be measured and studied by using standard test strips. In the workflow of the Blue Wool test, one reference strip set shall be stored protected from any exposure to light. Simultaneously, another equivalent test strip set is exposed under a light source defined in the standard. For example, if the lightfastness of the colourant is indicated to be 5 on the Blue Wool scale, it can be expected to fade by a similar amount as the strip number 5 in the Blue Wool test strip set. The success of the test can be confirmed by comparing the test strip set with the reference set that was stored protected from the light.

In graphical industry

In printing, organic pigments are mainly used in the inks, so the shifting or bleaching of the color of a printing product due to the presence of UV light is usually just a matter of time. The use of organic pigments is justified primarily by their inexpensive cost compared to inorganic pigments. The particle size of the inorganic pigments is often larger than that of organic pigments, thus inorganic pigments are often not suitable to be used in

offset printing Offset printing is a common printing technique in which the inked image is transferred (or "offset") from a plate to a rubber blanket and then to the printing surface. When used in combination with the lithographic process, which is based on ...

. In

screen printing Screen printing is a printing technique where a mesh is used to transfer ink (or dye) onto a substrate, except in areas made impermeable to the ink by a blocking stencil. A blade or squeegee is moved across the screen to fill the open me ...

, the particle size of the pigment is not the limiting factor. Thus it is the preferred printing method for printing jobs requiring extreme lightfastness. The thickness of the ink layer affects the lightfastness by the amount of pigment laid on the substrate. The ink layer printed by screen printing is thicker than that printed by offset printing. In other words, it contains more pigment per area. This leads to better lightfastness even though the printing ink used in both methods would be based on the same pigment. When mixing printing inks, the ink with the weaker lightfastness defines the lightfastness of the whole mixed color. The fading of one of the pigments leads to a tone shift towards the component with better lightfastness. If it is required that there will be something visible from the printing, even though its dominant pigment would fade, then a small amount of pigment with excellent lightfastness can be mixed with it.

References

External links

Doing your own lightfastness testsPupulandia: Onko taide ikuista – tai kuuluuko sen olla?
*{{cite journal , doi= 10.1177/004051756303300707 , title = The Lightfastness of Dyes: A Review, journal = Textile Research Journal, volume = 33, issue = 7, pages = 528, year = 1963, last1 = Giles, first1 = Charles H, last2 = McKay, first2 = Robert B Pigments Color Photochemistry Properties of textiles