It is fascinating to note that some of the techniques for shaping and colouring
glass were lost and rediscovered over the centuries. So that glass from Egyptian
or Roman times is more ornate and complex than early medieval glassware's.
Until  the  later  part of the 20th. Century, glass making was a closely guarded
skill. To a  certain  extent, it still is. As we no  longer  have  secret  guilds  to
apprentice  into trial and error still  seems  to be the main stay of learning to
work in glass. In Slumping, the glass is laid into, or  on  top of a mold and heated
just to the point where it "Slumps" to fit the form of the mold.  Once the  glass  
reaches the desired form it must be cooled quickly enough to stop the  
movement  that  will  result  in   cracking. Although these methods sound
simple, the  objects  created  are  quite  often  very  intricate in their design,  
and  hours of  painstaking  labour  may go  into  the arrangement  of the glass.
In many examples the glass has been fused into a pattern and then slumped to
fit a particular form.
     Another category  of  glass work  which  requires a  kiln involves the more
elaborates use molds to form glass into more complex shapes. Virtually any
shape  that can be formed  in clay or wax can  also  be  made  in glass .These  
more  advanced kiln  forming processes include kiln casting  like melting glass
into a mold  inside a kiln, called  "Pate de Verre"  or "Paste of glass" and glass
casting is a  pouring  molten glass into a mold.
     These processes tend to be  more  complicated  than  basic fusing  and  
slumping. They  require  working knowledge of basic fusing and slumping
techniques and processes. The finished product has  more  than one  colours and
the difference of them can be seen when the glass is viewed from different
angles. Technically  there are three colours in the finished piece transmitted
when light passes through the clear glass, reflected  the light that bounces off  
the  glass and  reflects a  second  colour,  with  the third  colour viewed  by
looking at the glass from a 45 degree angle.
    Adding different metal oxides to sand, soda  and  lime, the basic formula for
glass creates different colours. For example  greens  and  aqua glasses  usually
have iron,  while amber and brown colours are produced by  adding  small
amounts  of  iron  and sulphur. Light blues require copper, while dark blues
require  very  small  quantities of cobalt.
    Amethyst glass contains manganese. Opaque white can contain  either  tin  or
calcium. Selenium is one metal oxide that is used to produce reddish colours,
Some reds and pinks even have a bit of gold in them and he soft purples and
violets that gave the nickel oxide. Slumping  is  nearly  synonymous  with  
sagging,  however, slumping  usually  implies  a bending without noticeable  
change  in  thickness of the cross section of the glass. But by keeping  the  
temperature  as  close  as  possible  and  high  enough  to  allow  downward
movement an avoid stretching.
     Sagging is the downward sinking of glass caused by its own  unsupported  
weight  as the glass softness when heated.  Sagging  as  a  process whereby  the
thickness  of the glass cross section  changes noticeably due to stretching. The
edges that overhand a mold will turn up as the middle sinks down as the slum
progresses,  the edges fall and the mold fills and this is the reason why vent
holes are necessary  in all  non-porous  molds  and should be  placed  in  the  
areas  of the  mold fill  last,  the  center  of the bottom always fill first, therefore
is not the proper  place  for  venting air trapped between the glass and the mold.
     More slumped forms require to use of more than one  mold as a deep bowl for
example is just about impossible to slump in one  firing using a single mold for
that reason a series of different molds are used in order to maintain control of
the glass. Always as a rule you must take notes  of  the  different stages while
you are working also with your design of different colours because they absorb
heat at different rates.
     I hope all this information is useful. I want to share my experiences which
are challenging the rewarding. I learn from  my failures and continue to strive
to understand the properties of glass during the firing (heating). My own
observations have permitted me to develop personal techniques in glass forming
through fusing and slumping.
     I gently enjoy this glass techniques as a way for  artistic creativity.
Glass Technique Slumping
Tools for Slumping