I've now seen swirlies when using a glass rod to spread the emulsion, so I can't claim correlation with the nitrile gloves.
I sketched up a fault tree analysis to look at all aspects of the whole process. After eliminating the obvious non-causes, basically the fault tree was telling me that the source is either a contaminant (containers, coating equipment) or an issue with the process (glass prep, coating, cooling). That analytical conclusion even makes sense. I've also had coating runs without swirlies, so reviewing my notes helped me eliminate some other sources (like the kids) and reduce the failure mode testing to a few variables. So I did tthat yesterday (some things I kind of knew the answer, but wanted to explore coating process variables anyways). Here is a summary of the testing. In all cases ambient temperature controlled to 70F (and plates are also at that temp when they are coated ... that hasn't changed since I started coating glass two years ago, which was a potential cause I was able to check off without having to test).
Temperature: Control group emulsion applied at 101F. Variable group coated at emulsion temperatures ranging from 100F down to 79F. Quality of coating got worse below ~90F, degrading significantly with lower temperature, but no real "swirly" correlation. (saw swirlies and good coatings in both groups).
Coating technique: Hand, glass rod, tilting of plate, glass rod not scraping substrate. No correlation (saw swirlies and good coatings in both groups)
Subbing layer: With and without subbing layer. No correlation (saw swirlies and good coatings in both groups)
Pulled out a new syringe, looked at the plunger and noticed a viscous liquid apparent on the unused plunger. What! My guess is a silicon lubricant, which explains why the plunger was so easy to operate. So that leads into contaminant testing.
I washed the components of that syringe in dish soap and warm water as best I could and marked it so that I could coat plates with the cleaned syringe and with a newly opened, untreated syringe. A correlation! And a moderate-to-strong one at that. Swirlies were much less apparent with the cleaned plunger, but still not completely gone from test plates. It could be that I didn't entirely clean the silicon off. After the correlation testing, I coated a couple dozen additional plates for my own use, and used the cleaned syringe to do so. This morning I noticed that those plates look very good, so as the evening (early morning actually) went on, the plates got better possibly because the rest of the silicon cleared out from use. I need to switch brands of syringes to follow up.
Emulsion from different storage jars: I divide batches up into smaller ceramic jars for storage until I need them. In this way I don't continuously reheat all the emulsion in the batch. There's a potentially mild correlation. I switched to using just the clean syringe about the same time I broke out a new jar of emulsion, but unfortunately my notes aren't as clear as to exactly when two events happened. So I need to follow-up on this one.
Air circulation during emulsion drying. Mild correlation with plates closer to the fan vs. farther from the fan vs. plates with no circulation. Could be tied as well to switching to using the cleaned syringe, as plates from earlier in the testing were placed nearer the fan in my drying setup. Additionally, I've always used a fan blowing on the plates to help speed up the drying process so that hasn't really changed from the past few years. I will follow up on this as well.
The evidence strongly indicates contamination in the brand of syringes I recently started using could be the culprit. I had just bought new packs of syringes about the same time as the swirlies started showing up, and they are different than the brand I've used before. I might have been misled in earlier testing as the syringe would clean out from use, and then the swirlies could have come back when I switched to a new syringe. Plates coated solely with the "good" syringe were significantly better looking, and in fact most of them (the later ones as explained above) were entirely clear of swirlies. However, there are a couple of additional variables I need to follow up on, and the plates with no circulation still need to be examined once they are done drying. I'm also going to break open a new syringe and intentionally contaminate a plate with the lubricant I find on it.
I guess my question would be if there's potential for interaction between the emulsion and silicon lubricant? I'll find out when I do follow-up testing.
So bottom line is to never assume that what should be clean and contaminant-free actually is!
Some of the results were good news hidden in the bad "swirly" results .. the lack of correlation to actual coating technique (smearing with fingers, spreading rod, or tilting the plate around) was the big one, because it clears the path to adopting faster production techniques.
My lesson learned is not to touch the wet emulsion with my finger, glove or no! So I machined an acrylic puddle pusher for larger plates. For the smaller plates I ordered the type of glass angled rod that medical folks use to spread bacteria culture around in a petri dish. The gelatin thickness is controlled by the amount I lay down with a syringe.
