Ramping of Silver and Halide Flow in Run-Salt Emulsions

[mghis]

Hello to all.

I have been studying for a while the methods of emulsion making, even if merely out of curiosity, since at the moment I have not adequate equipment to start experimenting.

I know from patents, publications, and from informations published in this very forum that the growth of silver halide nuclei in modern "Run Salt" emulsion is usually performed by two or more jets, one for AgNO3 and the others for halides. These jets are adjusted to get the desired constant concentration of silver ion needed to get consistent and well shaped crystals.

I do not understand why the flow of these jets is ramped up during the process. The main concern in this phase is, in my understanding, avoiding renucelation and thus ensuring the consistency of shape and size of grains. Since the surface of the grains increases, I can see why flow can be ramped up without provoking renucleation. But I cannot see how increasing the flow can modify crystal shape and size (given that Ag+ concentration is not changed).

In other words, what would happen if, in the growth phase, pAg is controlled and keep constant without increasing the flows of both jets?

Thank you all for any explanation.

P.S.: Please forgive me for my poor English: I am not a native speaker.

 

[Photo Engineer]

Your English is just fine!

Ok, ramping up flow rate is not done to change crystal habit (provided vAg is kept constant). The flow is ramped up in most cases to keep pace with the growth rate of the crystal surface. Growing too slowly can cause defects due to stress on the grain surface.

Of course, if you change vAg or the mix of halides then the ramp upwards may not be needed, and in some cases an actual ramp downwards may be needed although this would be very rare.

In some cases, the final stage is a run of just Silver Nitrate alone at a steady rate or ramp upwards to top off the silver used (reach a certain number of moles precipitated), while at the same time adjusting vAg and surface halide ratio.

PE

 

[mghis]

Thank you very much for your kind response! You and this forum are indeed an astounding resource for learning a subject for which nowadays is difficult to find comprehensive explanations.