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Mixing RTV Silicone

Having constructed a pour box around our masters and estimated it's volume in the previous part of this article, you're probably itching to mix up the RTV silicone and slop it in. However this is a sure way to end up with a botched job as it's crucial that we avoid introducing air bubbles into the silicone.

As stated in the first part of this article, the product I use is called 'Ultrasil' and has a coloured hardener which makes it very easy to see that it is fully and uniformly mixed prior to pouring. This is very important if the silicone to cure properly.

However the real beauty to a coloured hardener is you can mix all of your molds 'by eye' using colour matching to determine when you've got roughly enough hardener into the mix to guarantee a set mold. This saves having to do precise measuring and is fast and reliable. I've mixed roughly 30 molds this way and only botched one early pour.

The only down side to this method is you tend to over use hardener which could lead to you running out. It's never happened to me and I suspect Barnes supply excess anyway in anticipation of most molders being lazy devils and mixing by colour. Varying the amount of hardener added will change the working time and final strength of the set RTV, but most of these products should have an ample working time anyway. Definitely check the MDS (material data sheet) for your products before mixing them though.

Finally we're ready to mix! Carefully pour the white rubber component into your mixing pot until it's been filled to the mark that we made in the previous part of this article. This can be a messy process and I find it handy to have a scrap of foamboard nearby to scrape the edge of the rubber pot clean before putting the lid back on tightly.

I believe RTV products like these can be degraded by exposure to air moisture so don't leave the lid off your rubber pot for longer than required. The shelf life of an opened pot is reduced too, so it's often a good idea to plan and pour a series of molds together in order to use up your can of silicone in a reasonable space of time.

Add hardener a fair amount at a time (remember that with Ultrasil you're aiming for a 1:10 ratio - check the ratio for whatever product you use) and thoroughly mix it in with your mixing tool. Stir, don't 'fold' the mix and avoid rapid stirring. The less air you introduce to the mix the better. Take care to mix in the white rubber on the sides and bottom of the pot too. I find it handy to use transparent or semi-transparent mixing pots for this reason. I have a cat and a three year old in the house so have an endless supply of plastic pet food containers and jelly pots that get seconded to the garage once empty and cleaned.

Mixing RTV SiliconeCompare your mix to any set mold you have handy. Mix in more hardener until you have a rough colour match to the set mold. Always keeping in mind that a mold that's seen a lot of either resin or plaster can get a little lighter in colour.

Of course if this is the first mold you're mixing then the above paragraph is a little useless. In this instance I'd recommend using a roughly regular shaped mixing pot (no curves or sloped walls) and dividing the space between the bottom of the pot and the 'fill' mark into 10 roughly similar proportions. Fill nine of those with white rubber and one of them with coloured hardener then mix thoroughly. If anything you want to be a little too generous with the hardener for your first mold, just to make absolutely sure the silicone will set. I'd also suggest your first mold pour be reasonably modest in size. For future molds colour match to your first one.

In the next part of this article we'll pour the silicone.

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