Education‎ > ‎

1. What is pottery?

POTTERY = RECONSTRUCTED ROCKS!

What would happen if you dumped a bunch of dirt from your back yard into an active volcano?

In most cases it would simply melt down and mix right in with the other molten rock.   

Why?

Many soils have roughly the same chemistry as the rocks they came from.  Most contain at least 50% of the glass forming oxide silica.

Why is Silica so special?

Silica is like a universal lego.  It can attach to itself at lots of different angles with a near perfect and strong fit.  It can surround and contain all the other elements that do not fit together so flexibly.  It is the glassy matrix that holds onto everything else.

So why isn't there more obsidian?

The main reason is that rocks usually melt and cool in massive amounts. This means they cool slowly, and long cooling times usually means crystal growth in a rock.  
Generally, the higher the silica content the slower it can cool and still stay glassy.  Many, but not all, obsidians  have a fairly high (over 70%) silica content.  Regardless, if you cool the same composition really slow and you can have a coarse grained granite.

So what else is in the Earth's crust besides silica?

Out of almost 100 known elements, only eight (8) make up 99% of the Earth's crust.  And since one of the eight (8) is Oxygen, and it combines with all the others, we can refer to them collectively as the seven common oxides that make up 99% of the Earth's crust.

There is an easy way to remember them and their approximate proportion in the Earth's crust.  Just remember the Rule of Seven. You can think of the seven days of the week starting with Saturday.  

EARTHS' CRUST AVERAGE in round numbers. 

Saturday     = Silica        61%      [ 6+1=7 ]  glass former

Sunday       = Alumina    16%      [ 1+6=7 ]  stiffener

Monday      = Iron           7%                [ 7 ]  colorant

Tuesday     = Soda         3-4%    [ 3+4=7 ]   flux

Wednesday = Potash      3-4%     [ 3+4=7 ]  flux

Thursday    = Calcia       3-4%     [ 3+4=7 ]  flux/crystallizer

Friday         = Magnesia  3-4 %    [ 3+4=7 ]  crystallizer/flux


What is the melting temperature for the average Earth's crust material?

About cone 9, or approx. 2350 degrees F.

It is no accident that this is right in the middle of the stoneware/porcelain range.  This temperature looks more yellow white than yellow orange in most kilns.