Note for MSE 257A at UA
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Date Created: 02/06/15
MSE 257 Spring 1999 Lectures on Organic Materials in Paintings 1 Some chemical terminology COVALENT BONDS DOUBLE BONDS SATURATED UNSATURATED POLARIZED HYDROPHOBIC HYDROPHILIC HYDROGEN B OND MONOMER DIMER POL YMER are formed by two atoms sharing electrons A covalent bond contains two electrons one contributed by each participant atom in the bond are form ed when two electrons are shared by each participant atom Altogether there are four electrons involved in the double bond Double bonds are stronger and shorter than single covalent bonds compounds are organic molecules that do not contain double bonds only single bonds between carbon atoms compounds are organic molecules that contain at least one double covalent bond or polar bonds are such covalent bonds where the sharing of electrons between the participant atoms is not equal In a water molecule HOH for example the bonding electrons between the oxygen and hydrogens are more likely to be found around the oxygen The oxygen is therefore said to be negatively polarized while the hydrogens are positively polarized means water fearing If a molecule is hydrophobic it is not likely to dissolve in water means water liking refers to the association of the two nonbonding electron pairs of oxygen with a positive polarized hydrogen a van der Waals type bond Hydrogen bonds are not covalent bonds and therefore they are much weaker However they play an important role in polymer chemistry because they are often responsible for the spatial conformation of polymers 7 their ability to form crystalline regions for example Important examples mentioned in class were hydrogen bonding between cellulose chains causing high tensile strength hydrogen bonding between helical strands of collagen protein boiling in alkaline water disrupts these hydrogen bonds and causes collagen to no longer be helical and to become water soluble hydrogen bonding between segments of albumen protein causing it to have a globular confirmation with hydrophilic groups projecting toward the surrounding water and hydrophobic groups hidden in the folds of the protein away from water a molecule that becomes the building unit of polymers two monomers linked to each other a large molecule built up by repetitive bonding of smaller molecular units the monomers Examples of polymers found in paintings cellulose a polysaccharide in wood linen cotton plant gums water soluble polysaccharides collagen a protein in gelatin glue and size albumen a protein in egg yolk and egg white ADDITION POLYMERS We call a polymerization an addition polymerization if the entire monomer molecule becomes part of the polymer example polyethylene CONDENSATION POLYMERS We call a polymerization a condensation polymerization if part of the monomer molecule is kicked out when the monomer becomes part of the polymer examples nylon cellulose MSE 257 Spring 1999 Lectures on Organic Materials in Paintings CHAIN GROWTH POL YMERS S TEPGROWTH POL YMERS CELLULOSE AMINO ACIDS PROTEINS COLLAGEN ALBUMEN In a chain growth polymerization monomers become part of the polymer one at a time 1 an initiator adds to a monomer to yield a reactive intermediate 2 the reactive intermediate reacts with another monomer to form a new intermediate 3 step 2 is repeated to yield a long chain The important features of this type of polymer formation is new monomers are only added to one end of the growing chain examples polyethylene polystyrene are polymers produced by reactions between DIFUNCTIONAL monomers These monomers have two reactive organic groups Each reactive group in the monomer reacts independently of the other with a reactive group from another monomer In a step growth reaction the growing chains may react with each other to form even longer chains This applies to chains of all lengths The monomer or dimer may react in just the same way as a chain hundreds of monomer units long examples nylon polyester proteins is the polymer primarily responsible for the structural strength of plants is a condensation polymer and astepgrowth polymer consists of around 1000015000 repeating units of DGlucose monomer DGlucose is a sugar or saccharide therefore cellulose is a polysaccharide is insoluble in water chains are hydrogen bonded to adjacent cellulose chains hence the high tensile strength of cellulosic materials crystalline regions is hygroscopic it absorbs moisture from the environment when its water content is lower than that of the environment are difunctional organic molecules They have a reactive amine and acid group There are 20 different common amino acids Amino acids are the building blocks or monomers of proteins are condensation and step growth polymers that consist of repeating units or amino acids Proteins are also called polypeptides because the bond connecting two monomers is called a peptide bond is a protein and the primary constituent of gelatin glue and size exists in skin and bone in the form of a triple helix where the three adjacent collagen strands are held together by hydrogen bonding It is insoluble in water in this form is extracted from skin and bones by prolonged exposure to boiling alkaline water The hydrogen bonds are ruptured between the individual collagen strands and the helices unwind Once this happens collagen becomes soluble in water is the primary protein in egg white and egg yolk and is responsible for their behavior as binding media is in a globular balllike form in fresh egg yolk and white This conformation is achieved by hydrogen bonding between different parts of the protein chain The hydrophobic groups are thus hidden in the interior of the globule while the hydrophilic groups point outwards and interact with the surrounding water molecules Albumen therefore is soluble in water in its globular form denatures when exposed to light heat and certain chemicals Such exposure destroys the hydrogen bonds holding the balllike conformation When the hydrophobic groups become thus exposed albumen ceases to be water soluble MSE 257 Spring 1999 Lectures on Organic Materials in Paintings OILS AND FATS DRYING OIL S VARNISHES are triglycerides they consist of three long chains of carbon and hydrogen attached to a glyceride head saturated fats are triglycerides that contain no carboncarbon double bonds in the long chains fats contain few or no carboncarbon double bonds and are therefore generally solid oils contain several carboncarbon double bonds they are unsaturated compounds and are liquids are oils with at least three double bonds per triglyceride unit They dry because the double bonds allow the crosslinking of chains Crosslinking refers to making connections with other long carbon chains Crosslinks in drying oils occurs if they are exposed to light energy and oxygen Drying oils used in paintings are linseed oil poppyseed oil and walnut oil are made from natural resins and are water insoluble belong to the chemical family of terpenoids damm ar mastic and copal are plant resins shellac is an animal resin 2 Some conservation terminology SOFTWOODS and HARDWOOD S QUARTER CUT or RADIAL CUT TANGENTIAL or SLAB CUT WARPING SPLITTING CRADLES CRAQUELURE DRYING CRAQUELURE softwoods have a simpler microscopic structure than hardwoods poplar a wood used for Italian panel paintings is a softwood oak a wood used for northern European panel paintings is a hardwood boards are the best for panel paintings because they have very little tendency for warping boards have a considerable tendency for warping is caused by anisotropic dimensional changes in wood Wood shrinks and expands to different degrees along its three primary dimensions tangentialradiallongitudinal 21 01 generally refers to at panels acquiring a curved cross section as a result of repeated moisture level changes When the expansion and contraction of wooden panels is restricted stresses may build up in the panels If wood yields to these stresses and breaks it is referred to as splitting Splitting often occurs in warped panels are horizontal and vertical slats fixed to the back of panel paintings The members are designed to slide freely and thus allow the expansion and contraction of the panels while keeping them planar Cradles may cause more harm than help in some cases especially then the slats interlock and restrict any movement of the panel refers to the pattern of cracks that run through the ground and paint layer of a painting These cracks form when the more rigid ground and paint layer cannot accommodate the dimensional changes in the more flexible wood or canvas support Craquelure on panels is more regular in appearance than on canvas paintings refers to craquelure restricted to a particular layer of paint It occurs when the top layer of paint dries much faster than the rest of the layers below This top layer becomes rigid sooner then the rest and when they eventually dry as well and shrink the top layer will not be able to accommodate the dimensional change and develop a cracks MSE 257 Spring 1999 Lectures on Organic Materials in Paintings COBWEB CRAQUELURE refers to craquelure developed on a canvas painting after it encounters a point force TRANSPARENCY of oil paint films increases with time The crosslinking of drying oils increases their density and their refractive index The increasing refractive index may approach the refractive index of the pigment particles contained in the binding medium which makes the paint film more transparent ie the underlying layers of paint may become visible PENTIMENTI plural singular pentim ento are parts of the composition that were revised and changed during the course of the creation of a painting Signs of the original composition may appear with time as the transparency of the paint film increases Abrasion of the paint film through cleaning may also contribute to their appearance
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