At the moment I’m working on a number of new posts, after quite a long time not being able to write because of teaching, and these will include .. methods of hollow casting in enclosed moulds i.e. not needing a pouring hole; basic working with Jesmonite; making a ‘strengthened’ silicone rubber mould i.e. not needing a supporting jacket .. all these are coming soon! But for the moment I’ve updated my modelling wax info page in the Materials section, because using the soft ‘Terracotta’ modelling wax for mould setup proved so successful during a recent Modelling, mouldmaking and casting course that I wanted to note it there, and include it as a post here! My additions to the modelling wax page also include the results of some tests I made on how to dissolve i.e. clean up modelling wax, and I’ve updated current prices etc., but here is a transcript of the part dealing with mouldmaking preparation:
Tiranti’s ‘Terracotta Wax’ is by far the best option I’ve found so far for mouldmaking setup .. i.e. either for embedding a prototype form half-way to create the first part of the mould, or for building temporary containment walls.
In the first place the wax is ready to use without too much ‘conditioning’ between the fingers .. it becomes very soft almost immediately. In the past I used standard plasticine (the Newplast type, in the long cellophaned packets) by default, and although this generally worked well enough it took a while to make soft enough to work with comfortably. Also, when it stuck to prototypes it was very stubborn, taking a lot of time and effort to remove it completely. Often (though not always) when silicone rubber was cured against it, residues of plasticine would remain like a crust on the cured silicone surface and again, although this could be removed it could take a lot of time and effort.
In a recent run of our Modelling, mouldmaking and casting course we gave all of the students ‘Terracotta Wax’ for embedding or setting up and on dismantling the first mould halves (in order to start the next) the wax came free like a dream and hardly any time was needed for cleaning up! I recommended a particular method though, which I want to illustrate here using photos of a similar setup I made yesterday.
The example above is, in this case, not the initial prototype form as you can see .. this has already been covered with silicone rubber and the setup here is for making the plaster or Jesmonite jacketing (also often called mother mould) to support the silicone part .. but this will still serve as illustration. The object first needs to be just securely rested on a baseboard in a sausage-ring of wax (note .. the baseboard must be large enough to accommodate all subsequent stages i.e. if this were the initial prototype the baseboard must be large enough for the silicone layer and then, later, making the harder jacket parts. Generally, for medium-size forms like this, 5-6cm space around should be enough). It is important that the object is just resting securely in the wax i.e. there is no need to press the wax forcibly into the object’s surface at this stage, just make sure that the object cannot move around.
This ring is then built up, as above, keeping the contact between object and wax to a minimum. Below, when the ’embedding line’ is reached, i.e. the line around the object which is meant to serve as a smooth barrier or containment wall, the topmost wax can be pressed and smoothed against the object’s surface. It only needs just a few mm of contact, as long as this layer is properly supported by the wax built up underneath. I’ve found that modelling wax, at least this particular one, can be smoothed much more readily with clean metal tools, tending to drag a little against the wooden ones. The most important factor at this stage is a ‘watertight’ seal between the wax wall and the object’s surface. This kind of barrier or containment wall just needs to be as smooth as shown i.e. certainly not perfectly!
As with the baseboard, a little forward-thinking is needed. The wax platform needs to be wide enough all-round for whatever will be applied to it i.e. in the case of starting with the initial prototype, both the silicone layer and then the subsequent mould jacket. But, unlike the baseboard, at least the platform can be extended later if need be.
Below, I have pressed natch marks in the wax around the form using a ball-headed modelling tool (these can often be found in modelling tool packs for cake decoration). These natch marks should not be too deep! Whatever material is going on top will reproduce these marks (and transfer corresponding ones to the other side of the mould when made) and they are simply there to locate the finished mould halves together properly. I’ve found that the modelling wax sticks much less to tools than plasticine, though about the same as Super Sculpey.
The final task in this setup is making a small vertical containment wall around the natch marks. For medium-size forms around 1cm distance from the object is sufficient. In this case building a wall was necessary because I was planning to start the mould jacket by pouring Jesmonite over the form.
‘Terracotta Wax’ sticks very readily to itself, much more so than either plasticine or especially Super Sculpey, partly perhaps because of its extreme softness! It was only necessary to press this wax strip lightly onto the platform to establish a proper seal. I’ve found the most effective wall is made by flattening a rolled strip of wax first (by pressing and turning over a few times) and then trimming the edges to a flat right-angle with a scalpel.
On the whole no release coatings are needed with this wax. It releases well from a variety of materials even if rough or porous, as long as it’s not deliberately worked into the surfaces. Most importantly silicone rubber will cure and detach cleanly from it. I have found that if Jesmonite is poured onto it and the mix is ‘polymer rich’ i.e. towards 2:1 powder to polymer liquid (the standard recommended for economy is 3:1), the wax can adhere to the cured Jesmonite in some places. But if this happens the wax is very easy to scrape off.