Viscous between amorphous particles

Viscous forces in sinter bridges between amorphous particles. 

There is a practical maximum concentration of amorphous silica that can be dispersed in the aqueous silicate solution. It is often desirable to incorporate as high a concentration of amorphous silica as possible, yet still have a workable fluid binder to apply to the sand. If the proportion of amorphous silica to soluble silicate is too low, than the shake-out will be adversely affected. On the other hand, if the ratio of amorphous silica to soluble silicate is too high, the mixture will be too viscous and must be thinned with water. Also, there will not be enough binder to fill the spaces between the amorphous silica particles in the bond, and it will be weak. In generally, the higher the content of amorphous silica relative to sodium or potassium silicate, the weaker the initial bond as set by carbon dioxide. Conversely, the more silicate in the binder, the higher will be the initial and retained strengths. 

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