Our masters are set up in the pour box and in the previous part of this article we covered the art of mixing up the silicone. We're now ready to pour the mold.
Resist the temptation to just slop it in and leave it to set as this will almost certainly lead to trapping air bubbles in your masters and related annoying blobs appearing in any casts you make. "Act in haste, repent in leisure" as they say, and remember that air bubbles are the garage caster's nemesis.
Without fail, I always brush the first coat of RTV silicone over the masters by hand. It won't guarantee you have no air bubbles in your mold but it will significantly reduce the chance of it happening.
So take a deep breath, load up your brush with mixed RTV and start painting it over your masters. You'll need a good stiff brush (e.g. hog's bristle) because RTV silicone is a very thick liquid. You should have ample time to carefully paint each and every master in your pour box before the silicone is anywhere near the end of its working period. But check your product's instructions to be sure!
Once you're done, put the brush aside in a cleaning fluid (RTV silicone cleans up in painter's turpentine), and make sure you come back and thoroughly clean this brush once your mold is fully poured. It's a little ironic while working up this tutorial I was so busy taking photos I completely forgot about the brush I'd used until the next morning. A brush loaded with set RTV is a throw away. This is also why I use cheap hog bristle brushes for all my RTV molding and plaster casting work: because you can throw them away! I buy a packs of 10 various brushes from a local '$2 shop'.
Finally, take your remaining RTV silicone and slowly pour it into the box to fill your mold. I generally try to pour in at a corner or edge and let the thick fluid 'roll' across the masters to cover them completely. I believe this should introduce less bubbles than say sluicing it all over the mold. Keep a close watch on any parts of your masters that might be inclined to trap air as you pour.
Another tip with pouring is to make sure the rim of your mixing pot is clean. You've just mixed the rubber and hardener in there and there may be some unmixed components around the rim of the pot which you do not want to pour into your mold.
While pouring you may notice there's a little colour variation in the silicone as it goes in. Usually this is obvious because you can see the 'ribbon' of silicone you've just poured laying on to of the RTV already in the box. Don't worry about this too much unless it's very clear you have unmixed rubber at the bottom of the pot. If so take a moment to regret not completely mixing the components before pouring. Then stop pouring and try gently stirring up the RTV in the mold with a brush or tool to mix it in with the rest. You can also try to recover the partially mixed rubber left in the pot by adding a little more hardener.
It's also worth noting that RTV silicone is 'self adhering' which means you can later pour additional silicone into a box partially filled with cured silicone - providing the set silicone is reasonably clean and free from dust etc. I wouldn't recommend making a habit of it, but you can use this to recover botched pours where something has gone wrong.
And you're done! Stand back and admire your handiwork for a few minutes. Pay particular attention to the edges and seams of your pour box because if you notice RTV escaping anywhere you'll have to deal with it now. The truly paranoid might like to keep a blob of Klean Klay handy just in case they need to shore up any gaps or holes. This hasn't happened to me yet but you never know...
Make sure the mold is placed somewhere level and out of the way to set. Level is very important because eventually you want to turn this mold over and pour resin or plaster into it which means the bottom has got to be flat and level. Out of the way because there's normally a long setting time involved. Again check your product's instructions for details. Then wait at least twice as long as that before you go back to it. Seriously, I usually pour in the evenings and wait 24 hours for a product with an 8 hour recommended setting time. Do not be tempted to mess with your mold until you're sure it's set. RTV silicone is a deceptive product - the surface can appear set while the interior (next to your masters) is still gooey. Imagine your surprise at discovering that while trying to peel the mold off! Yes I speak from bitter experience.
There are three techniques I use to avoid this issue:
I realise this last test is quite subjective but believe me it works. Cast a handful of molds and you'll likely see at least one that does this when you expect it should be completely set. Be patient and give it another day of curing time. If you've mixed the silicone well enough that at least the surface has set then eventually the interior should set too.
If you finally give up and peel the mold away from the masters only to discover some or all of the interior has failed to set then it's likely that your rubber has been contaminated or has aged (consider throwing the batch away) or that you constructed your masters from some material that retards the RTV setting process. As I mentioned earlier, sulphur containing plasticine can have this effect, and while that's the only mastering material I know to avoid, there may be others. Having said that, I use a lot of DIY products, epoxy putties including green stuff, resin pieces and Plasticard for my masters without difficulty.
When you're sure it's set, you're ready to proceed to the final part of this article and finish off the mold. You did remember to clean out that brush didn't you?