From: Jan Bentz 
To: Bob May 
CC: atmlist 
Reply-To: Jan Bentz 


There is a process called "contacting" where two pieces of glass
are actually in partial molecular contact.  They are not wrung together but
meticulously cleaned at least as well as for aluminizing.  The two surfaces
must fit well enough for non-circular interferance fringes to be seen.  The
technique is something of a black art and requires almost infinite
patience.  The pieces must be able to float on one another.  If the
smallest resistance is felt they are not clean enough and must be cleaned
again.  The actual contacting is started at one point by pushing down with
the finger.  If it works this point will spread to the entire area and the
fringes will disappear.  The edge is sealed with shellac to keep out water
and air.
  The joint is sensitive to thermal shock but this may be due to
dissimilar materials.  They are separated with a hair dryer blowing on one
of the pieces.  When they come apart there is a loud crack.  They adhere
well enough to withstand polishing and fairly rough handling.
   I have no hands on experience with this but have seen it done with
large pyrex (24"x3") disks with  .5" thick BK-7 plates
contacted to them and then figured flat.

    Jan Bentz


Bob May wrote:

>The joint isn't very strong although the large area does cause high
>pressures to happen when taking them apart.  As a result, the glass will
>eventually come apart.  Even wrung together polished glass will eventually
>pop apart over time.  If you do get a good full joint by wringing then you
>can then pop that into a furnace and get them to fuse together without any
>air between the two pieces but usually some air happens and you have a
>bubble layer.
>Bob May
>http://nav.to/bobmay
>bobmay{at}nethere.com
>NEW! http://bobmay.astronomy.net
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