Monday, November 22, 2010

Resistive notes (2) - to shake or not to shake

Ah, that is the question.  Many emerging chemigramists have written to ask me, when I lay my photo paper in a tray of developer or fixer, do I agitate it there, in its 10 mm or so of liquid, or do I let it rest?  Does it even matter?  Let's approach this question by thinking about it for a moment.

It seems clear that a motion tangential to the surface of the paper (and of the liquid too, if not deep) will produce a shear stress acting on the paper in a manner to dislodge anything clinging there, if given enough time and enough agitation.  Now, to review, chemigram materials resident on the paper might include (1) hard resists, like varnish, (2) soft resists, like syrup, or (3) quite inviscid and nonadhesive fluids like the fixer or developer themselves, for example, or other household chemical agents that may be chosen for suspected properties.  But their rapid dislodgement is not always desirable; it really depends on the effect we seek.  In the case of inviscid, free-flowing fluid, we are usually quite content just to dip and pull, securing the signature mark of the enveloping chemical and moving on to other things.  But just for the record we show, in figure 1, what a shear stress might yield in such an evanescent situation, from one of my own works from 2009.  It's rather hard to reproduce, this effect, requiring a certain lightning-quick flick of the wrist, since the viscosity coefficient of waterlike materials is so small - so don't attempt this if you discourage easily.  Just think about it as a possibility.

figure 1
The choices and our ability to profit from them are more varied as we move up the rheology scale to tougher resists.  Some workers may favor a slow progressive erosion of the bonds between paper and resist, others may like it faster.  The difference between the two can be seen in the width of the resultant Mackie lines.  Testing this, we performed a simple experiment three times during the first week of November, using Ilford RC paper, somewhat impure Kodak developer and fixer (but normal for chemigrams), Golden MSA varnish, and common x-acto incisions, to compare a paper that received monitored agitation with one that was left unattended in the tray.  The time to withdrawal was twenty minutes, by which time the effects were amply demonstrated. 
figure 2a, agitated

We illustrate this by one of the runs - they were all similar - shown in figure 2a (with agitation) and 2b (stationary).  When we enlarge them and apply a scale (not shown in this post), we find a Mackie expansion of 10 mm in the stationary case vs 14 mm in the agitated one, a 40% increase that remains significant even if the crudeness of our measurements is discounted.

We can compute a velocity, for those who care, of 4.2 x 10 (exp -2) m/hr.  Not much, but not zero either.

figure 2b, stationary

At this rate, the Mackie line would circle the earth at the equator in a runtime of 3 x 10 (exp 8) hours, or 34,224 years, give or take.  So shaking the paper seems a good way to get where you want to go.  Patience helps too.