THE EFFECT OF COALESCENCE ON AGBR TABULAR GRAIN FORMATION

In the precipitation of tabular AgBr crystals in aqueous gelatin solut ions, experimental results reveal a strong correlation between the deg ree of flocculation during nucleation and the resulting tabular grain fraction. In addition, large changes in the final tabular crystal popu lation can be produced by changes made solely in the nucleation step, indicating that in such cases twinning occurs during nucleation. Becau se in the presence of gelatin, flocculation always precedes coalescenc e, it is concluded that coalescence plays an important role in twinnin g and the generation of tabular crystals. Possible coalescence mechani sms for twinning are discussed. It is also demonstrated that high popu lations of tabular AgBr grains can be obtained at various nucleation p Br conditions, thus raising questions about the validity of AgBr twinn ing mechanisms invoking solution properties that prevail only at low p Br. The results of this study also suggest a turbidimetric method for monitoring, controlling, and generally optimizing the formation of tab ular crystals during the nucleation step.