Glow in the dark (GITD) toys have an additive that stores light energy and then slowly releases it over time, causing the toy to glow.

This is usually done by adding an encapsulated powder to the silicone. Encapsulated glow powders minimize risk but at a trade-off, creating a grainy glow effect and increasing the likelihood of flops. They come as either a zinc sulfide or strontium aluminate chemistry. Zinc sulfide is the older type, dating back to the 1930s and has a shorter glow time of under an hour; while strontium aluminate is a newer chemistry that can last well over three hours.

Risk and Encapsulation 💚

GITD pigments are fairly low risk. Neither chemistry is carcinogenic despite their glowing nature. Zinc sulfide based powders are slightly toxic (Health risk of 1) when ingested and have no known target organs. Strontium aluminate powders are considered chemically and biologically inert and pose no health hazards. Oddly, it is only zinc sulfide that has any FDA approval for use in cosmetics and then only for skin contact; not around lips or eyes. Because of this, a company claiming to sell FDA approved glow powder is potentially a yellow flag and warrants a little digging.

Strontium aluminate powders have no known hazards, so the un-encapsulated powder may be used. However, many makers choose to use the encapsulated version to give customers a second layer of reassurance about the pigment. Zinc sulfide powders should be used as an encapsulated powder whenever possible and rub out tests performed regularly otherwise.

This encapsulation is not to be confused with the encapsulation provided by the silicone. Encapsulated pigments are tiny microspheres of inert plastic filled with a pigment and are produced in a number of different ways, including coacervation, freeze drying, direct coextrusion, fluidized bed coating, spray chilling, and more. Depending on the process; a high percentage of the pigment will be successfully encapsulated, but not all of it. Some microspheres may be incomplete and expose the contained pigments, which argues for the importance of performing a rub out test when using a new pigment or process to verify sufficient secondary encapsulation by the silicone. Encapsulated powders will usually be sold as "water proof" and may not say encapsulated in the listing. Try to find an SDS sheet (previous nomenclature was MSDS) since the encapsulation material may itself, rarely, be an irritant.

The larger risk from GITD powders is the rub out of particles, leaving tiny pits in the toy which can inhibit sanitization by the customer. Always perform a rub out test with new pigments and new processes. A small amount of rub out is normal, but extreme rub out should be tracked to its sources and eliminated.

Image shows acceptable rub out VS flop level rub out.

Technical Usage Tips and Tricks 💚

GITD powders are dense and sink rapidly. In thin and slow-to-jell silicones like Ecoflex 00-30, this can turbo flop whole runs of toys. To reduce sinking:

• Use a thixotropic enhancer like Thi-Vex to increase viscosity
• Gently stir silicone a second time after degas to redistribute powder that has sunk
• Use only as much powder as is needed and do not overload the silicone. There must be sufficient silicone between each particle to be structural
• Allow silicone to "cook" for a few minutes before pouring by leaving it undisturbed to jell slightly, and then gently stir to redistribute powders
• Pour a protective skin layer of silicone. Near Clear can be used without compromising the look of the pour

GITD powders are often hydroscopic and will start to clump as they absorb moisture from the air. To reduce clumping:

• Store bulk powders in airtight containers. Large bags can go straight into a mason jar
• Only decant as much powder as will be used in a week from bulk storage to smaller containers
• Only open bulk containers inside and away from humidity
• Flush bulk containers with canned air before sealing
• Gently stir silicone a second time after degas to break up powder that has sunk

General Tips:

• There have been some anecdotal reports of cure inhibition with some GITD pigments. If buying from a new vendor, perform a cup test with a fairly heavy load of pigment to check for inhibition
• Do not buy from unknown vendors on marketplaces like eBay and Amazon. Consistency is an issue with such vendors and micron sizes and densities of powders may change drastically, leading to unexpected flops across entire silicone runs
• Glow powders may be mixed with other pigments like Silc Pig Electric to create new colors, but luminescence will be reduced
• Silicone with GITD powder in it will try to sink under silicone without. Be cognizant of this when the workshop is cold, you are using a thin silicone, or using a pot life extender like Slo-Jo
• Liquid pigments like Silc Pig Electric have a stronger effect on changing the perceived color of glow than mica
• Mica blocks / lessens glow effects more than liquid pigments

Perform a rub out test whenever you:

• Use a new GITD Pigment for the first time
• Use a new silicone for the first time
• Have large temperature swings in the shop
• Change pot life or viscosity with additives

Future topics:

Types: Pros / Cons

Table for specific powders and ratings