The Mechanism of Sintering

 "Do glass molecules actually migrate when they are sintered together? "

Sintering occurs at the atomic level, where the atoms at the edge of the particles attach to others in other particles. An analogy occurs to me of Scottish country dancing. In big gatherings, small groups are formed to perform the dance, say an eightsome reel. As the dance goes on the groups become more coordinated and eventually form one large group, held together by the people on the edges of each group.

A more scientific description comes from Wikipedia:

“Sintering … is the process of compacting and forming a solid mass of material by heat or pressure without melting it. … 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.”

Applied to glass this means that you can make a solid piece of glass out of multiple touching or overlapping pieces that do not change their shape. This uses low temperatures and very long soaks.

 Schematic-diagram-for-the-sintering-and-fusion-reaction-of-the-glass-frits-on-a-substrate.
Credit: ResearchGate

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 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 atoms of the molecules time to combine with their neighbours in other particles.

Sintering occurs in the range of 610°C to 700°C (1130°F to 1275°F). The lower limit is determined by the strain point of the glass and by practicality. The length of time required at the strain point - 540°C/1005°F - is so long (days) that it is impractical.

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. The process of crystallisation breaks the bonds already formed between the atomic structures of the molecules. These studies showed that the onset of devitrification is at 650°C/1204°F and is visibly apparent at 700°C/1292°F regardless of the glass used.

The lowest practical temperature for sintering is 650°C/1203°F. Indications are that at least an additional two hours are needed for the sinter soak for each 10°C/18°F reduction below 650°C/1203°F. This would make for a 12-hour soak at 610°C/1131°F. For me this is not practical.

More information on the kilnforming processes and sintering experimentation is available in this eBook: Low Temperature Kiln Forming.

Low temperature breaks in flat pieces

The usual advice in looking at the reasons for breaks in your pieces must be considered in relation to the process being used.  Breaks during low temperature processes need to be considered differently to those occurring during fusing.  

The advice for diagnosing breaks normally, is that if the edges are sharp, the break occurred on the way down in temperature. Therefore, the glass must have an annealing fracture or a compatibility break.  It continues to say if the edges are rounded it occurred on the heat up, as it broke while brittle and then rounded with the additional heat.

This is true, but only on rounded tack and fused pieces.

I exclude low temperature tack fuses from the general description of when breaks occur in flat pieces as it is not applicable at low temperatures.  

Low temperature flat work includes sintering, laminating, sharp profile tack fusing, etc.  There are lots of other names used for this "fuse to stick" work.  In all these cases, the finished glass edge will be barely different than when placed in the kiln.  It stands to reason therefore that you cannot know when the break occurred, as the edge will be sharp whether it broke on the way up or the way down.  

Periodic observation during the firing is the only way to be sure when the break occurred. These observations should coincide with the move from the brittle to the plastic stage of the glass.  Therefore, about 540C.  It can be at a bit lower temperature, but not a lot.  If the glass was not broken by that time, you can be fairly certain it broke on the way down.

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