Glossary

Acicular – Needle-like or whisker-like crystalline structures.

Aliphatic Molecule – An organic compound in which the constituent atoms are joined in straight chains, branched chains or non-aromatic rings; they may contain single, double or triple bonds (i.e., saturated or unsaturated).

Annealing – A heat treatment that alters the properties of a material (typically a metal), increasing its ductility and reducing its hardness, to improve its ability to be worked.

Aromatic Molecule – An organic compound which contains one or more ring structure with fully conjugated bonds; this allows certain electrons to disassociate across the ring structure (πbonding), granting increased stability compared to similar non-aromatic structures. Benzene is the most common aromatic compound.

Auxochrome – A functional group which modifies the observed colour of a chromophore.

Azo Compound – A molecule containing the diazenyl group (R–NN–R′); many of these materials are strongly coloured and so are important in dye chemistry.

Barkcloth – A non-woven fabric made from the inner bark of certain trees (usually of the Moraceae, or mulberry, family), produced through retting and pounding.

Biopolymer – A polymer that is produced by (or derived from) a living organism.

Chelating Agent – An organic molecule capable of forming multiple bonds to a metal ion, often used in conservation to remove soiling through the formation of water-soluble ring-like complexes that can be washed away.

Chromophore – The region in a molecular structure which leads to its colour, due to its electronic structure, usually resulting from extended series of conjugated bonds in aliphatic and aromatic compounds.

CIE – An organisation (Commission International de l’éclairage) that establishes methods of colour measurement. The 1976 CIELab system relates mainly to a colour space but the more recent CIE 2000 formula is a calculation that uses CIELab values but brings colour differences closer to the perception of the human eye.

Complex – A molecular entity, formed of a central atom or ion (frequently a metal ion) surrounded by a specific number of loosely bonded ions or molecules in a particular geometry.

Conjugation – A sequence of alternating single and double bonds in aliphatic and aromatic organic compounds.

Copolymer – A polymer containing more than one species of monomer.

Degree of Polymerisation (DP) – The average number of monomer/repeat units in a polymer molecule.

Dendrite – Branchlike structures of crystals which form within metal as it cools and solidifies from the molten phase; in multicomponent systems the composition of the dendrites may differ from that of the bulk.

Detrital – Sedimentary rocks composed of rock debris or fragments.

Earlywood – Wood formed early in the growing season of a tree, comprised of larger cells with thinner cell walls, and forming the lighter part of the annual growth ring (cf. latewood); also called ‘spring wood’.

Efflorescence – The formation of crystals on the surface of an object, as the result of the migration of a mobile species (such as a soluble salt or a plasticiser) from the bulk to the surface.

Emulsion – A mixture of two mutually insoluble liquids, in which one liquid is found as a dispersion of fine droplets within the other.

Flavonoids – Naturally occurring vegetable pigments named after the compound flavone, commonly used to produce yellow dyestuffs.

Fluorophore – A chemical species that exhibits fluorescence due to its electronic structure (cf. chromophore).

Flux – In the context of glass and ceramics, a substance that helps lower the melting point of the main constituents.

Functional Group – Specific chemical structures or regions in a molecule that are responsible for its chemical reactivity.

Glass Transition – A physical process that occurs in amorphous materials as temperature increases, characterised by a transition from a hard and brittle (glass-like) state to a soft, viscous (rubbery) state. The glass transition temperature (T_g) is the temperature range over which this process occurs.

Green Body – A mass of weakly bound clay material (often with organic or inorganic additives), before it has been fired, sintered or vitrified.

Hygroexpansivity – Change in dimensions due to variations in moisture content.

IR – Infrared, referring to electromagnetic radiation in the region of the spectrum adjacent to visible light, and of a lower frequency (longer wavelength): wavelength 700 nm–1 mm, frequency 430 THz–300 GHz.

Isomer – Compounds containing the same atomic composition but displaying different atomic arrangements, resulting in different chemical and physical properties.

Lamellar – Alternating layers of different materials producing inhomogeneous microstructures, formed naturally or through material processing.

Latewood – Wood formed late in the growing season of a tree, comprised of smaller cells with thicker cell walls, and forming the darker part of the annual growth ring (cf. earlywood); also called ‘summer wood’.

Lees – Dead yeast cells, left over from the fermentation process.

Libriform Fibres – Highly elongated cells, with a high proportion of lignin in the cell walls, found in xylem, which contribute to the hardness and resilience of wood.

Lixiviation – The separation of a mixture of insoluble and soluble materials by dissolving the latter in water or another suitable solvent.

Macromolecule – A large molecule formed from linked monomeric units. The term is often used interchangeably with ‘polymer’, although it applies specifically to the molecular species rather than the bulk material.

Masstone – The full colour pigment or coating, when applied thickly.

Minimal Intervention – A conservation ideal in which the minimum of treatment is performed to maintain the stability of an object.

Monomer – A molecule capable of bonding with other monomers to form large chain-like or networked molecular structures (polymers), through the process of polymerisation; the basic building block of a polymer.

Mordant – A substance (typically a polyvalent metal ion) which enhances the affinity of a dye for a substrate and improving its fastness; the use of a mordant may also alter the colour of the dyed material.

Mullite – A needle-like silicate mineral, formed during various melting and firing processes, and found in porcelain. Also known as porcelainite.

Mucilage – Hydrophilic polysaccharide secretions from certain plants, often gel forming, such as those originating from aloes.

Native Metal – A metal that is found in its pure, metallic form in nature, rather than as a mineral.

Oligomer – A molecule comprised of a few monomers; dimers and trimers (two and three monomeric units, respectively) are examples of oligomers.

Passivation – A process leading to the reduction of the reactivity of a material (often a metal) to its environment, though the formation of an inert or minimally reactive surface layer.

Perspex – A transparent polymer made of poly(methyl methacrylate).

Plasticiser – Low molecular weight species, which can be added to polymers or other bulk materials to modify their properties, typically to increase plasticity and decrease viscosity (thus enhancing flexibility and durability).

Plasticiser Migration – The tendency of plasticisers in a bulk material to travel within that material over time, often forming sticky or crystalline deposits on the surface. The process generally leads to the embrittlement of the original object.

Polymer – A large molecule (macromolecule) comprised of many monomeric units linked together by a polymerisation reaction. It can also refer to the bulk material formed from polymer molecules.

Polysaccharide – A class of polymers including cellulose, starch and chitin, which are formed from simple sugars (monosaccharides, such as glucose, fructose and arabinose).

Quenching – The rapid cooling of a material to prevent the formation of large crystals, by which desirable physical properties are promoted, typically hardness.

Radical – An atom, molecule or ion with an unpaired valence electron; these species are normally highly reactive.

R Group – An abbreviation used to denote the part of a molecule outside the immediate region of interest.

Retting – A process in which vegetation is left in water for an extended period during which microbial and chemical processes break down substances holding cellulose fibres together, rendering them easy to separate and subsequently use. The term is most frequently encountered in the treatment of flax to obtain linen fibres.

Saturated Compound – An organic molecule which contains only single bonds.

Scission – The breaking of a chemical bond.

Sinter – To fuse a powder or particulate material to into a cohesive mass, through the use of heat or pressure, without causing it to fully melt; the resulting structure has a network of small cavities and is often porous.

Size – A substance applied to a material to improve its absorption or handling characteristics. Sizing of paper reduces the ability of the material to absorb liquid, allowing inks or pigments to remain on the surface rather than bleeding or spreading; it may also alter other properties. Sizing of textiles or yarns improves strength and abrasion resistance, reducing breakages during manufacture.

Stereochemistry – The aspect of chemistry concerned with three-dimensional (spatial) arrangement of atoms and molecules, and the way in which this geometry influences their interactions and reactivity.

Stoichiometry – The ratio of quantities of species involved in a chemical reaction, allowing the calculation of reagents and products in such reactions.

Surrogate Material – Substitute or replicate materials that are often employed when original objects cannot be sampled or tested, and whose properties and behaviours are chosen or modified to mimic those of such objects.

Tempering – A slow heat treatment performed after quenching to increase toughness.

Tracheids – Highly elongated cells found in the xylem of plants, which allow the transport of water and soluble material such as salts.

Tristimulus Colour Measurement – A method of measuring and representing colour using three values, such as the CIE L*a*b* system.

Unsaturated Compound – An organic molecule which contains at least one double or triple bond.

UV – Ultraviolet, referring to electromagnetic radiation in the region of the spectrum adjacent to visible light, and of a higher frequency (shorter wavelength): wavelength 10 nm–400 nm, frequency 30 PHz–790 THz.

Valence – The number of electrons that an atom uses in bonding; a measure of the ability of an atom to form bonds.

Viscoelasticity – The mechanical characteristic of materials that exhibit both instantaneous, elastic behaviour and time-dependant, viscous behaviour when placed under stress.

Vitrification – The transformation, through heat, of a material into a glassy solid.

Wavenumber – A measure of frequency, normally defined as the reciprocal of the wavelength in centimetres, with the units cm⁻¹; frequently used in vibrational spectroscopy.