Re: When water is frozen, both white and clear ice is formed, why?

Have a close look at a grain of sugar. It is clear. But when you put a lot 
of grains of sugar together it looks white. And the finer it is ground up 
the whiter it is: icing sugar looks whiter than caster sugar which is 
whiter than ordinary sugar.

The difference between white and clear has to do with light scattering from 
the surfaces of grains. The finer the grains, the more surfaces, and the 
more something looks white than clear.

But what has all this to do with ice? We are not talking about snow, which 
is certainly white rather than clear because it has lots of grains, but 
about ice that freezes in a single lump. Well, it is not only grain 
boundaries that can scatter light. The inclusion of small amounts of other 
stuff in ice can make irregularities in the structure that will scatter 
light. Any area where there are a lot of moderately large scale 
irregularities in the structure of an ice block will look white. 'Large 
scale' means bigger than the wavelength of visible light, which is about 
1/2000 of a millimetre, or half a micron. Smaller irregularities cause a 
different type of light scattering which can make ice look milky and 
bluish.

There are three main causes of these irregularities: dirt, trapped air 
bubbles, and cracking due to mechanical strain as the ice is freezing. If 
you look at some of the whitish bits of ice with a good magnifying glass, 
you might just about be able to see whether you are looking at air bubbles, 
dust and dirt, or stress cracks.

How to reduce the amount of white ice? Make sure you have very clean water, 
with no dissolved salts. Boil it to get rid of the dissolved air which is 
the cause of most of the air bubbles in ice (air that is dissolved in 
liquid water will not stay dissolved in the solid ice that forms, but makes 
bubbles as the ice is forming instead; similarly salts will usually make 
grit particles instead of staying in solution).

That just leaves the problem of stress cracking. You might have to 
experiment with that one. I do know that fairly thin sheets (half a 
centimetre thick or so) usually show much less stress cracking than solid 
blocks. The other thing that may be important is the speed of freezing, but 
I do not know whether faster or slower is better.