Physics Photo of the Week

November 20, 2009

Zone Refining
This is a photograph of my raingauge that had been frozen during the night of January 30, 2009.  What caused the brown blob in the center of the frozen rainwater?

Normally we think of rainwater as quite pure.  However, birds have scattered sunflower seed shells from the nearby birdfeeder.  Some of these dark sunflower seed shells were sitting in the bottom of the raingauge.  Some of the tannins, or dark pigments, dissolved in the rainwater.  As the stained water in the raingauge froze, the freezing began on the outside of the cylinder.  The freezing freezes the water, but not the stains that were in solution.  The stains propagate inward remaining in the liquid water, and the perimeter ice is much more pure than originally.  This process is called "zone refining".  As a result the stains, still in solution, propagate toward the center and become trapped as a spherical region of dark material. 

The photo at right shows the water in the raingauge after it was allowed to thaw.  The solution of the tannins has diffused in the bottom region of the liquid.

This process of zone refining is analogous to purification by distillation.  Two compounds in a solution can be separated or purified by boiling - one component boils at a lower temperature than the other, so the vapor is much purer than the original solution.  In zone refining, the purification results from a difference in the melting temperature of the two components.

Zone refining is also very technologically important in refining ultrapure silicon to manufacture transistors, integrated circuits, and computer chips.  An ingot of Silicon is melted in one particular location.  The melt is propagated along the ingot by means of a special furnace.  With repeated passes of the melt along the ingot, the impurities move with the melt, leaving a very pure ingot of Silicon.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to 

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