The cool UV light.

Well, i'm also wondering the same thing.. Some of the photography tutorials here or others say that they are using flash when taking picture
But some say that uv from the flash can harm the resin..
Anyone can explain to us?

 

Yellowing is a natural part of the resin aging process. Your resin doll will yellow, no matter what you do. Camera flash will not speed up the yellowing of your doll and flash is not harmful to your doll. While resin is affected by UV light and heat, both of which are emitted by some built-in flashes or external flash units, the reality is that a camera flash fires so quickly and is at a low enough level of heat and UV that your doll's actual exposure is minimal. Nowadays, not all camera flashes are UV-emitting anyway. Besides, if camera flashes were emitting dangerous amounts of UV, they wouldn't be safe for people, either.

You'd have to be firing a flash directly at your doll constantly for days to expose it to enough UV and heat for 'rapid yellowing' to occur. It's okay to take a doll outside for a little while for photos in the sun as well; it's just not a good idea to expose your doll to constant sunlight for long periods of time. Camera flash is safe and can be very effective in creating lovely, natural effects in your photography.

 

All the flash photos you will ever take of your doll will not add up to the UV exposure of just 1 second in direct sunlight, so I wouldn't worry at all about that. Also, polyurethane yellowing is actually a mix of different chemical changes, some of which can happen with just UV light, and some of which happen by reaction with oxygen even in complete darkness. Temperature speeds up the oxidation reactions too. The ideal storage conditions for a doll to minimize yellowing would probably be in a cool, dark place devoid of oxygen (e.g. flushed with an inert gas like pure nitrogen). This last part isn't practical (though it is common for museum storage of things), but cool and dark will go a long way, and you can limit oxidation by making sure any long-term storage of the dolls is in airtight containers. Sealing the doll with a spray like MSC also helps because the MSC yellows sacrificially by absorbing a lot of the UV and sealing the underlying polyurethane from exposure to oxygen as well--the doll will still look yellowed, but most of it will come off with alcohol, since it's the MSC layer that is yellow, not the doll's plastic itself.