This is one of those projects I've had on the back burner for quite a while. You know the kind of thing, where you see someone else have a go and think "I really must try that at some point..." So I did (admittedly after having purchase the materials over 12 months ago).
The process is simple, easy enough to do at home and the vast majority of the effort is in making the master as I discovered. There is plenty of reference material for the whole process on the web. Ultrawerke offers a particularly good 5 part tutorial on his blog.
You can buy the materials over the net or from specialist outlets. I've won't go into details about the types of mold making rubbers or resins because there is plenty of info on the web for that too, including Wikipedia. Most are two part compounds which are inert until mixed. The ones I used are below.
|Ultrasil two-part RTV rubber|
|Ultrasil two-part resin|
Making a master
This is the hardest part. I made two 4 inch sections of wall and it took a good 3+ hours with a couple of restarts. The stones are sculpted from blue high density foam / thin sheet foam and glued together with PVA. Simple enough but getting them consistently the same size and also in scale with 28mm miniatures is more difficult that I expected. Even after a while tweaking they are slightly different widths and heights but it shouldn't be noticeable once painted and based. I based them on lolly sticks to give a nice flat base for both casting and use. When finished I sealed them with 50/50 PVA and water. I don't know if that helped but it didn't harm the process so I'll keep doing it.
|Two stone wall masters in foam and wood|
Making a form
I went for the seemingly ubiquitous Lego form work which brings me to a slight digression. In any given body of Lego there will be approximately 70% bricks and 30% foreign bodies through past use. This breaks down roughly into:
10% - sand and pet hair.
5% - other toys including model cars, game pieces, dinosaur parts and unidentifiable plastic.
5% - old nut shells and lolly wrappers.
5% - cutlery (presumably for prising bricks apart).
5% - other household objects including fridge magnets, batteries and most likely ANYTHING else you've been looking for which is 3" square or smaller.
And here it is with the masters lightly blu-tacked in place.
I discovered afterwards that mine was a tad too large so I wasted some RTV but this is a first try so what the hell.
Making the mold
You have to measure the two parts of the RTV compound very carefully to ensure a good result. Too much or too little of one part and the mold may not set or be fragile. The proportions are easy enough with digital kitchen scales but getting the total required amount is challenging. You can fill the mould with sand or something else and then measure that material or take pot luck like I did. I had to add two more mixes but it sets slowly so no harm done. Once poured in the form you need to leave for a period and being an impatient guy for this kind of thing it's TORTURE. Fortunately football and chocolate filled the emptiness. ;-)
Once the RTV has set I peeled away the Lego and removed the masters. One master came apart but surprisingly one survived entirely intact. Neither left anything behind so all good.
Mixing the resin is much simpler. It's a 50/50 mix so two transparent plastic cups is perfect. I measured the volume of one wall with water and then used that amount as the required volume for each part of the compound. Simple. It sets faster than you'd believe but you still have time to take care when pouring. I tapped the mold lightly for a few seconds but could see almost no air bubbles.
|First batch on the go|
|See where the RTV left a whole when not mixed properly. No harm done.|
Waiting for the casts to cure is where even greater patience is required but thankfully not nearly as much as the RTV. Exactly one shower and a cuppa worth of patience in fact. The mold proved to be very flexible and the casts robust enough that they caused no issues like tearing or breaking when being removed. Nice one!
The finished article
Here it is. The first cast. A few minor bubbles but these easily convert into flaws in the stone to personalise each piece. I'm stoked with how well these worked given I've never tried this before. In two hours I've made another 4 batches and will do enough for 12 sections I think. Next I'll make a corner piece, end piece and ruined section.
|Cast above, master below|
Stay tuned for the painted and based versions...