Having set up our masters in a pour box in the previous part of this article we're almost ready to start pouring the RTV silicone. However RTV silicone is expensive stuff so we don't want to waste it. Neither do we want to pour it only to discover that we didn't mix enough. So can we tell how much to mix?
The easiest way to estimate the volume is simply to pour some other fluid into the assembled box and tip it back out into our mixing container. In fact the ideal substance, because it leaves nothing behind that needs cleaning up, is fine sand. I nabbed mine from Whangamata several summers ago but you may prefer another beach or even a builders merchants. Be sure to wash and dry it before the first time you use it.
Take your sand and fill the pour box with it. Gently shuffle the box around as you go in order to level the sand. Aim to get it so you've just covered the highest point of the masters, then add another 4-5mm (1/4") of sand to give a good thickness to what will be the base of the mould.
Once you're happy that the sand sufficiently fills the pour box, gently pour it out into some large container being careful not to lose any onto your work area. Now pour from the large container into your mixing pot and mark the level to which the sand fills the pot. This is how much of the basic rubber component you'll need to fill your pour box. Remember while doing this that the down side of using a little more silicone than is absolutely necessary is that it costs a little more while not using enough could result in a weakened mould that gets ruined - err on the generous side.
Tip the sand out of your mixing pot and brush out any extra grains. Be sure to clean any extra grains of sand out of the pour box too. I use a combination of gentle shaking, blowing and brushing off the masters with a hog's bristle brush. It'll only take a couple of minutes to clean up and you'll be ready to start mixing the silicone as we'll describe in the next section of this article.