Art, Archaeology & Analysis March 10, 2003 Alison Shull Dept. of Materials Science & Engineering

Transcription

1 Art, Archaeology & Analysis March 10, 2003 Alison Shull Dept. of Materials Science & Engineering

2 Today s Handouts from: Color Mixing Guide Handpainted Samples On paper sheet On transparency

3 Describing a Color (Yet another color system) Munsell System of Color Notation: H V/C Hue (color) Value (lightness) Chroma (saturation) Munsell Color Tree

4 Spectral Reflection Reflectance % Wavelength [nm] As a rule of thumb, the proportion of the reflectance curve filled by light in each section of the spectrum indicates the relative level of response by the R, G and B cones

5 What would you see? (A) eflectance (B) % R (C) Wavelength [nm] (A) Titanium white, (B) Ivory black, (C) Hansa Yellow

6 Colorant Pigment - Paint Colorant: Any substance that imparts color to another material Dyes Do NOT scatter light Absorb certain wavelengths, transmit others Soluble in water Pigments Insoluble dispersed particles Absorb and/or scatter particular wavelengths Paint: Pigmented liquid Designed for application to a substrate as a thin layer that some time later converts to a solid layer Pigment: solid portion of paint Vehicle: liquid portion of paint in which the pigment is dispersed, composed of binder, thinner and additives Binder: nonvolatile portion of liquid vehicle» binds pigment particles together» binds paint layer to layer beneath

7 Pigments Inorganic Pigments Metallic Compounds Oxides, sulfides or salts of metallic elements such as iron oxide, cadmium sulfide, titanium dioxide. Known as earth colors (Ochres, Siennas, Umbers) or heavy metals (Cadmiums, Cobalts and Titaniums) Natural (mined & refined) Synthetic (chemically manufactured) Lightfast Moderate to high degree of opacity (exception: zinc oxide) High load Produce relatively low saturation, opaque, matte paint with low tinting strength Organic Pigments Metal free, based on complex carbon chemistry (carbon, hydrogen, oxygen, nitrogen). Examples: Quinacridone, Naphthamide, Phthalo, Hansa and Anthraquinone Synthetic organic pigments are lightfast (Some natural organic dyes can be chemically converted into pigments (such as madder lake), but are fugitive (not lightfast)) Translucent Low load Produce relatively high saturation, translucent, high gloss paint with high tinting strength On the GOLDEN Acrylics Color Mixing Guide foldout: Inorganic: Titanium White, Zinc White & Yellow Ochre Organic: Quinacridone Magenta, Naphthol Red Light, Hansa Yellow Medium, Phthalo Green & Phthalo Blue

8 Inorganic Pigment Copper Sulfate: CuSO 4

9 Color Mixing Look at the comparison of the same picture, painted with (a) Inorganic pigments (b) Organic pigments

10 Comparison of Natural & Synthetic Single Pigment Paints Quinacridone Gold Burnt Sienna Natural earth color: Burnt Sienna Modern synthetic colorant: Quinacridone Gold 25 % Reflectance Natural inorganic pigment: Calcinized natural iron oxide Synthetic organic pigment: Quinacridone Wavelength [nm] Hue: 3.0 Value: 3.25 Chroma: 4.0 Hue: 6.0 Value: 2.50 Chroma: 5.5

11 Single Pigment Paints: Draw downs Natural Inorganic Burnt Sienna Synthetic Organic Quinacridone Gold Masstone on glossy white Masstone on matte white Masstone on glossy black Undertone on glossy white If %reflection of paint over white background is the same as %reflection of same paint over black background, then the paint (at that thickness) is opaque.

12 Multiple Pigment Paint Mix#2: Single pigment paint desaturated with gray paint Mix#2 Pyrrole Orange + Transparent Gray (mix of bone black and zinc white) = 5 : 1 Amorphous carbon Zinc oxide

13 Another Multiple Pigment Paint Mix#1: Mixture of three single pigment (synthetic organic) primary color paints = Mix#1 Quinacridone Magenta 2 : 0.06 : 4 + Phthalo Blue + (Green Shade) Hansa Yellow Medium

14 Comparison of Spectral Reflectance for Mix#1 & Mix# % Re fle cta nce Wavelength [nm] Mix#1 Mix#2

15 All 4 Brown Paints Quinacridone Gold Burnt Sienna 25 Quinacridone Gold Burnt Sienna 70 Mix#1 20 Mix#1 % Reflectance Mix#2 % Reflectance Mix# Wavelength [nm] Wavelength [nm] The average height of the reflectance curve corresponds to value Lower saturation (lower chroma ) colors have flatter reflectance curves For much more on reflectance curves, pigments and color vision go to

16 The choice of pigments is what causes the forger the most trouble. Taft & Mayer, The Science of Paintings, p.88

17 History of White Pigments Calcined bone or white clay (kaolin): Cave paintings in France ~30,000 years ago Lead White (lead carbonate) One of oldest man-made pigments: fill lead pot with vinegar (acid), bury in manure pile (source of heat), disintegrates into white powder Main drawbacks: (a) Toxic (b) Turns brown when exposed to sulfur fumes Only white oil color available until middle of 19 th century. Partially replaced in 19 th century by zinc white, almost fully replaced in 20 th century by titanium white. Zinc White (zinc oxide) Advantages over lead white: (a) Not toxic (b) Zinc sulfide is white Used in paintings since late 18 th century, but is translucent Titanium White (30% titanium dioxide (TiO 2 ), 70% barium sulfide (BaSO 4 )) Advantages over lead white & zinc white: (a) Not toxic AND (b) Inert (c) Greater opacity (hiding power, coverage) due to higher refractive index Development driven by commercial applications, not need for better artist s paint

18 Hiding Power Hiding power: Measure of a paint s ability to cover a surface opaquely so that the color of an underlying layer cannot been seen in visible light. Depends on Absorption Scattering (Difference in refractive index of pigment and binder) Size of pigment particles Density of pigment particles Linseed oil n = 1.48 Zinc white (zinc oxide) n = 2.01 Lead white (lead carbonate) n = 2.04 Titanium dioxide (rutile) n = 2.71 Increasing n=n pigment -n binder Would you want high or low hiding power in (a) house paint? (b) whiteout? (c) sunscreen?

19 History of the Use of Titanium White Pigment Year History 1791 The element titanium discovered 1821 Titanium dioxide studies 1900 TiO 2 as yellow enamel opacifier 1913 Norwegian TiO 2 pigment patent 1919 Production of anatase/baso 4 pigment 1919 House paint (cream color) 1923 Anatase artist s paint (anatase/baso 4 ) 1925 Anatatse artist s paint (anatase/caso 4 ) 1926 First white anatase pigment 1930 Gradual acceptance by artists 1934 Suggested for watercolors and tempera 1939 Production of rutile TiO 2 /BaSO 4 pigment 1941 Production of rutile TiO 2 /CaSO 4 pigment 1942 Suggested for oil-based paints 1957 Production of high quality (pure rutile pigment) From Table 8.1 in Taft & Mayer, source: W. McCrone, 1500 Forgeries, The Microscope, Vol.38, pp (1990)

20 Sources of Color Interaction of light with matter to produce color: Absorption Scattering??? For good descriptions and examples of 15 causes of color go to

21 Structural Color : Interference Interference of light beams reflected from the front and back surfaces of a thin parallel film Explains the interference effect as a source of color Explains how the interference effect is used to make pigments and paints

22 Synthetic Interference Pigments Color flip : Abrupt change in hue as observer changes viewing position Do not get confused with the terms: Interference Iridescence Pearlescence They get used differently by different paint producers but all mean the same thing: color due to the interference effect Why does the background color (white or black) change the color that you see?

23 Designing Paint There s more to making paint than choosing a pigment! Choice of pigment Choice of binder Choice of additives Optimize pigment to binder ratio, plus additives Color strength Resistance to marring, cracking Surface sheen Lightfastness Workability (viscosity (thickness), rheology(flow)) Controlling microbes (avoiding mold, mildew) Controlling working time (time until dry) Wednesday: Mark Golden of Golden Artist s Colors - his company custom designs acrylic paints for artists!