MadSci Network: Chemistry


Date: Tue May 23 23:41:44 2000
Posted By: James Griepenburg, , Chemical consultant, Chemmet Services
Area of science: Chemistry
ID: 956687845.Ch


Believe it or not, white, gray and black are the same "color".  They share 
a common property: They reflect or transmit all the wavelengths of visible 
light equally or at least with the same response as your eyes.  Notice I 
said your eyes not our eyes.  Hoping that you are wondering a bit I will 
give you a couple of references that were just mentioned elsewhere in the 
Mad Scientist, two books by Marcel Minnaert,  "Light and Color in the 
Outdoors” and the classic "The Nature of Light and Colour in the Open 
Air".  Both are available from and are probably in your local 
library.  These books are marvelous!  After reading them you will look at 
everything from a fantastically new viewpoint. Also the Kodak "Here’s How" 
series on [mostly Color] Photography gives simple, precise, and 
interesting descriptions of color, pigments, vision and photography.

A white material or pigment both reflects and/or transmits almost all, 
usually more than 90% or so, of all the wavelengths of light falling on 
it.  A material such as a polished metal, which reflects almost all of the 
light, is not white because it doesn't transmit any light.  The black 
areas in a black and white photo are usually [they can also be platinum or 
palladium or other metals] very small silver particles.  These reflect but 
do not transmit light so the very fine particles end up absorbing light 
and they look gray or black depending on the depth of the particles.  A 
white pigment in the bulk such as a large crystal or a sheet of clear, 
colorless, glass or plastic reflects a small amount of the incident light 
and transmits the remainder.  If the material is ground into fine 
particles there are more reflecting surfaces and more light is reflected 
backwards by a process known as diffuse reflection.  The material 
becomes "white".   As the particles become smaller and approach molecular 
size they lose the ability to reflect and again become colorless.  Two 
examples:  An ice cube, if properly frozen is clear; snow, which is small 
crystals, is white; molten snow which consists of water molecules randomly 
arranged is again clear [and almost colorless. pure water does have a blue 
color].  A large salt or sugar [rock candy] crystal is clear and 
colorless.  The sugar and salt you buy are small clear crystals but look 
white in bulk. Powdered sugar doesn't look clear at all.  When you 
dissolve sugar, giving dissolved sugar molecules, or salt, giving 
dissolved sodium and chloride ions, in water; the solutions are again 
clear and colorless.

A white pigment also has some other important physical and chemical 
properties.  It must be chemically inert, usually insoluble in water and 
capable of being ground to a fine powder or suspended as small droplets.  
It must also be obtainable in a pure chemical state so it doesn't 
fluoresce or phosphoresce, that is give off light different from the light 
shining on it.  There is another useful property.  It should have a high 
refractive index, a high ability to bend light that strikes it obliquely.  
Some materials that are used as or act as white pigments: Paper, flour, 
powdered sugar, whipped cream, whipped egg white [meringue, mayonnaise], 
whipped fat [Crisco], water [clouds, fogs].  These are useful but do lack 
the property of chemical stability or insolubility so they are usually 
used only in food or simple play crafts.  There are many white materials 
that are more chemically inert and are used as pigments.  Common ones are 
silica[SiO2], chalk[calcium carbonate[CaCO3],  Titania[titanium dioxide, 
TiO2], Zirconia[Zirconium dioxide, ZrO2], baryta[barium sulfate,BaSO4], 
gypsum[calcium sulfate, CaSO4], powdered glass, zinc oxide[ZnO] and zinc 
sulfide[ZnS].  Probably the most common in modern paints are silica and 
titanium dioxide the first because it is inexpensive the second because it 
has a high refractive index and makes paint more opaque.  Both are very 
chemically inert.  Pigments that are not used any more because of their 
toxicity are lead carbonate[PbCO3, white lead] and lead oxide[litharge, 
PbO].  These have high opacity or covering power because of their high 
refractive index.

A pigment works by the process of diffuse reflectance.  The finely divided 
pigment reflects incident light back towards the source.  This means that 
if one looks at the transmitted light from the other side it looks gray.  
The best examples of this are clouds; when the sun shines on them they are 
brilliant white; when they get between you and the sun they can be a 
dismal gray.  Photographic paper, which is designed to reflect a lot of 
light, can also look gray by transmitted light. .

There are also physical, physiological and psychological aspects to 
color.  The physical: A white material reflects almost all of each the 
light wavelengths incident on it.  This is not restricted to white light.  
A striking example is working under an orange photographic safe light.  I 
once brought a small beaker of potassium dichromate [a bright orange 
material] into a dark room and placed it on the table and went about 
setting up and of course letting my eyes and brain become accustomed to 
the light.  When I went to look for the beaker I couldn't find it.  All 
there was, was a beaker containing a bright WHITE material.  I finally had 
to turn the room lights on to check and the chemical turned orange right 
before my eyes.  A material that reflects all of the light incident on it 
looks white even if it isn't white under other lighting conditions. A blue 
object in blue light looks white; a red object in red light looks white.  
White is dependent on the lighting conditions, tungsten light, sunlight, 
fluorescent light, each gives its own character to white.  

Each person’s eye[s] and most photocell types respond differently to light 
wavelengths so they have different whites.  The eye brain combination 
adapts and after a time makes the overall color distribution look white.  
Photographic film doesn't do this so different lighting will give 
different colorcasts to a photo.  Finally there is a strong mental 
aspect.   Some things you simply expect to look white. Your brain either 
makes them white or you can become confused when they don't look white.  
For example, skin tones under mercury vapor lighting will look garish and 
even frightening. 

Color, even white, is a fascinating subject.  Go out and look for the 
differences in light and in the colors of things.

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