Sintering

This is a process used in glass to stick glass together without any change in appearance of the separate pieces.  It has various names - fuse to stick and lamination are two.

General description
“Sintering or frittage is the process of compacting and forming a solid mass of material by heat or pressure without melting it…. Sintering happens naturally in mineral deposits [and] as a manufacturing process used with metals, ceramics, plastics, and other materials.

“The atoms in the materials diffuse across the boundaries of the particles, fusing the particles together and creating one solid piece. Because the sintering temperature does not have to reach the melting point of the material, sintering is often chosen as the shaping process for materials with extremely high melting points such as tungsten and molybdenum….
 
“An example of sintering can be observed when ice cubes in a glass of water adhere to each other, which is driven by the temperature difference between the water and the ice.”
https://en.wikipedia.org/wiki/Sintering
 
Applied to glass this means that you can make a solid piece out of multiple touching or overlapping pieces that do not change their shape.  This is done by using low temperatures and very long soaks. 
 
The usual process is to take the glass at a moderate rate up to the lower strain point.  The rate of advance is slowed to 50°C or less to a temperature between slumping and the bottom of the tack fuse range.  The operator must choose the temperature, largely by experimentation. 
 
The slow rate of advance allows a lot of heat work to be put into the glass.  This, combined with a long soak (hours), gives the molecules time to combine with their neighbours in other particles.
 
Sintering can be done in the range of 610°C to 700°C.  The lower limit is determined by the strain point of the glass being used and practicality.  

The upper limit is determined by the onset of devitrification. This  has been determined by the scientific studies of sintered glass as a structure for growing bone transplants.  Devitrification reduces the strength of the bonds of the particles at the molecular level.  These studies showed that the onset of devitrification is at 700°C and is visibly apparent at 750°C regardless of the glass used.  Therefore, the choice was to use 690°C as the top sintering temperature. 
 
For reasons of practicality the lowest temperature tested was 650°C.  Indications were that at least an additional two hours would need to be added to the sinter soak for each 10°C reduction below 650°C.  This would make for a 12-hour soak at 610°C.  For me this was not practical.
 
My recent testing has indicated some guidelines for the sintering process:
 
The ramp rate has significant effects on the strength of the resulting piece. 

• A moderate rate (150°C) all the way to the sintering temperature needs a two-hour soak at the top temperature. 
  
• A rapid rate (600°C) - as used in medicine – to the sintering temperature requires approximately six-hours soaking.
• A rapid rise to the strain point followed by the slow 50°C per hour rate to the sinter temperature requires a three-hour soak.

 
The temperature range of 610°C to 700°C can be used for sintering.  The effects of the temperature used have these effects:

• With the same rates and soak times, lower temperatures produce weaker glass.
• The lower the temperature, the longer the sinter soak needs to be for similar strengths.  Generally, the soak at 650°C needs to be twice that of sintering at 690°C.
• Lower temperatures produce more opaque glass.  In this picture all the glass is clear powder and fine frit in the ratio 1:2, powder:frit.

 

The annealing of sintered objects needs to be very cautious. The particles are largely independent of each other, only joined at the contact points.  The annealing soak needs to be longer and the cool slower than for simple tack fusing. 

• Testing showed that annealing as for 12mm is adequate. 
  
• There was no advantage of annealing as for 25mm as that did not increase the strength.

 
Porosity
Although the structure of the sintered glass appears granular, it is not porous except at or below 650°C.  At the lower temperatures, the glass becomes damp on the outside and weeps water.  At 670° and 690°C the outside became cool to touch but did not leak water.  This observation depends on evenly and firmly packed frits.
 

Grain structure at 650C

Grain structure at 690C

The keys to successful sintering of glass are the use of a heat work through slow ramp rates, and long soaks throughout the whole firing.

Further information is available in the ebook Low Temperature Kiln Forming.