I’ve been doing experiments to find various light blocking materials that didn’t settle. I started with dyes since @tdixon1000 already found a solution with Colorcon ink, but found them to be suboptimal as the only light blocker because the right balance to achieve light blocking but not substantially increase exposure time is pretty tricky.
The one problem with Colorcon ink is that it is difficult to source in reasonable quantities, since 8 pounds of ink is multiple lifetimes worth. I tracked down an MSDS for NT12 and the formulation is pretty simple, just carbon black and a glycol ether. Glycol ether is also commonly used in dyes and has the property of being both a wetting agent (forms smooth dispersions with powders w/o clumping) and broadly miscible with just about anything.
I sourced some carbon black from Amazon, but was pretty disappointed. It settled quite quickly. Then I ran across some courseware about sediment settling in water and came across a simplified form of Stoke’s Law for computing settling velocity. That insight from this is that while the difference in density between the powder and liquid is a linear coefficient to the settling velocity, the particle size is a squared term. This opened the door to experimenting with some powders I hadn’t previously thought to try.
Dry lubrication uses powders of varying types, typically graphite, molybdenum disulfide, tungsten disulfide, or PTFE. The lubricating behavior of the powder is impacted by its fineness, so these powders are usually labelled with their fineness in micron particle size, as opposed to carbon black pigments which aren’t very strictly measured.
Graphite is the cheapest, but I couldn’t find anything finer than 44 microns. Molybdenum disulfide is available at a fairly reasonable price at 1.5 micron fineness, but is over 5g/cm^3, pretty heavy. But, remember that particle fineness is a squared term, that dominates significantly over the difference in density.
It appears to work. I tested a concentration of 0.007 to 0.011 grams for 10mL of resin (the range is a bit of imprecision with my scale).
1.5 micron molybdenum disulfide powder after 24 hours:
1.5 micron molybdenum disulfide powder after 48 hours
This stuff is easily sourced from Amazon at:
http://www.amazon.com/gp/product/B00ICXS2OS
It also comes packaged in a matter that makes it easy to dispense. Powders are annoying to handle when them come in bags, and fortunately this comes in a bottle with a dispensing tube that makes it easy to dispense.
I also tried it both directly mixed into resin and first mixed with glycol ether. Adding the glycol ether makes it easier to handle, otherwise the powder has a tendency to stick to the mixing container, however did not otherwise effect the settling properties. I will probably pour glycol ether into the bottle to make an ink dispersion and reduce the possibility of having a dusty accident since 1.5 micron powder is probably not good to inhale.
If you do try this, let me know how it works for you!