The finished plaster mold

After the last post on making the Imp mold, only one large side remained. Here he is boxed up once again so I can make that final plaster piece.



This is the complete mold fresh out of the mold box.



All my pieces separated clean. In the foreground are the two large side pieces, and behind are the front gusset and the poll piece. The rubber original is still seated in the back gusset. The only piece not yet removed in the small insert under the tail.



Because that piece is flush against the larger side piece, I need a notch so I can lift the piece off the casting. As this picture shows, I did get a notch cut, but I misjudged the border of the inner piece and chipped the edge. What I wanted was a notch to the outside of the piece so that I didn't make my inner piece any smaller. It should still work, but it's even smaller (and easier to drop and lose!) than before.



The final plaster mold is there on the left, and the reassembled rubber master mold is on the right. The rubber master can now be used to make another plaster mold, repeating the same steps I took to make this first one. The plaster molds are only good for 15-20 castings (not all of which will survive to the final glazing), so it's important to have a "mold of the mold" so I can make more of them.

And that's exactly what I will do if this first mold works well.



The final step is to round all the outside edges and corners and bevel the edges where the pieces meet. This tells me at a glance that this is a final production mold. It also keeps the pieces from chipping.

As I said in a previous post, this mold has been drying for more than a week now. As of today, I am pretty sure it is dry. My timing is perfect because I leave for a trip to the beach this morning, so I can't try it yet. That means that it will be really, really dry when I first get a chance to test it. I need to know if the mold will work, and nothing will spoil a mold like trying to pour slip into it too soon. My goal was to make it impossible for me to jump the gun!

Wishing everyone a Merry Christmas!

Meanwhile, the mistake I didn't avoid

As I noted in a previous post, the plaster mold for Imp is done and drying. What I have been working on, while posting the process for Imp, has been the mold for his mother, Vixen.

When I was a brand new graphic design student, I had a teacher explain how I had broken one of the rules of good design. Aha! If there were rules, there must be a book! Where could I find the rulebook, I wanted to know. Surely someone had written them down so the rest of us wouldn't have to learn the hard way (like a public critique in front of a classroom of one's peers). My teacher explained that the rules weren't written down because true mastery of one's craft came when you knew which rules to break and when to do so.

I have often thought of this when I have taught seminars on horse color. Giving the basic rules of pattern expression is important because students need some kind of framework to begin to make sense of the subject. But much past a basic grasp of patterns, rigid attachment to "the rules" actually gets in the way of deeper understanding. It's much too easy to learn a given fact, and give it more importance than it should have.

Which is exactly what I did with my Vixen mold.

With Imp, I was introduced to the general rule of thumb of "tighten the mold perimeter as much as possible." I hadn't done this with Imp when I first boxed him, so I had to go back and correct the problem. Not wanting to make the same mistake twice, I made sure the area around Vixen was a minimal as possible. No unnecessary displacement here!

Unfortunately what I forgot was one of the other rules of moldmaking, which is "if the two big sides don't touch at each corner, the mold won't work as the box for the smaller pieces.



See how the corners on the two big pieces touch? That makes it possible to pour the gusset pieces withouth boxing the mold. If the gusset piece extended all the way to the left corner, the mold would have to be boxed in order to contain the pour.


The blue-tinted areas show the outline of what would be the inner pieces. The bright green line shows where the other gusset pieces will extend. With this size mold, more than 80% of my perimeter will be made up of innner pieces. That won't work!

With Vixen I created an even bigger problem because the areas involved were rather large. What's worse, I didn't realize this until it came time to clay up the inner pieces. So I already had one large side poured all wrong.



So I'm going to try some creative salvage work with my Legos, and see if I can't fix my error without claying her up again.

Pouring the first large side piece

In order to do the next pour, the first of the two big side pieces, I need to box the mold back up again. I didn't get the pour spout flush with the sides, however, and the tight fit of the Lego box caused me some concern. I didn't want to take a chance that the pressure might shift the original ever so slightly.



I decided old fashioned wooden mold boards, and a slightly looser fit, were a safer bet.



Here I have sealed any gaps with plasteline and poured the plaster.



With the plaster cured, I have unclamped the mold boards. On the top is my first large side piece, then the gusset, and then my remaining rubber side (with its plaster backing).

Here is my plaster mold so far, with the rubber original inside. I just have one piece left to pour now.



In the next step the only rubber left in my plaster mold is the original. I'll remove that and replace it in the rubber master (pictured behind the plaster) when I'm finished.

The mistake I avoided

As I said in my previous post, I almost made a really big mistake with my mold. Although I had expected a number of technical problems with this particular piece, everything had worked out perfectly. My first pull - the large side that comes off first - released easily without disturbing any of the other pieces. That's important since jarring the inner pieces can stress the legs, causing them to break later when the piece is demolded or cleaned. Even more important, I had good registration on the legs. Nothing had shifted during my pouring.

I began to wonder if Imp might not prove less complicated than I originally feared. Perhaps this first version would work and I would never need to make the more complicated second version. (The one where he gets cast in pieces and is then reassembled.) Here he was, looking really good with a five-piece mold! That's the really basic type of horse mold; two sides, front gusset, back gusset, poll piece. I was so excited by the idea that I might get away with making one of Sarah's horses with just those five pieces, that I almost tried it.

Which would have been a big mistake, and I would have realized it the first time I tried to demold him and ripped off his tail. The mold needed a sixth piece.



I had already seen the necessity of this when I drew my mold lines. The undersides of his tail and fanny recess too deeply for the second large side to pull away. To capture those details, I would need a tiny inner piece. Because pieces like this are small and don't extend to the outside border of the mold, they cannot be poured like the gussets and poll piece. They have to be made by hand for each plaster mold. It's fussy, time-consuming work, but the worst thing about them is that there isn't a corresponding rubber piece for them.

That means if you just replace the rubber pieces one-by-one, without thinking about it, you will forget to make them. I've done this more often than I'd like to admit, but not usually with the very first test mold.

Thankfully I did remember it before I went any further.



Here I've built up a clay barrier to contain the plaster under his tail.



I want my plaster to look like this for the next step.



Here I've spooned a small amount of plaster into the area. I'll need to add some more plaster, but I need it to set a little more before I do that.



When the plaster looks like this, I can add some more. I want to add a little height to my piece so I have something to work with when it comes time to shape it, so I need the plaster firm enough that it doesn't just spill over the barrier.



Here I've dabbled a bit of the firmer plaster onto the top of what I applied before. Now it's time to wait for the plaster to firm a little more.



When the plaster looks like this - pastey but still damp - I can shape the piece.



If the plaster is at the right stage, the clay barrier can be removed and the plaster will retain it's shape.



The plaster should retain it's shape, but still be soft enough to shape with light pressure. Here I've used a minarette to clean up my mold lines and plane the piece so it will pull later.



After I have the basic shape right, I use a soft, slightly damp brush to smooth the piece. I will also need to clean any excess plaster or plaster dust before I pour the side piece.



Here are all my finished and sealed inner pieces. To the right is the rubber master, which I have begun to reassemble as I finish with each piece. (I had not yet cleaned the last bits of plaster dust from the pieces on the left when this picture was taken.)

Now all that's left are the two big side pieces.

The other inside pieces

In the last moldmaking post, I had just finished and sealed this first inner piece. That's my mold soap in the small tub with the blue lid, and the brush I use to apply it is there in the foreground.

In the next few steps, I'll be making the rest of the inside pieces. First, however, I'm going to need to anchor some of this little guy's floppy bits. Thin, floppy legs and tails are the biggest challenge when making plaster molds of small-scale models because they have too much give. This means the legs (or tail, or in Imp's case even his pour spout) can shift when the plaster gets poured, and the final castings will be distorted.

This is where the Tacky Glue comes in. A tiny spot inside the mold cavity will hold the legs, tail and pour spout in place. The glue isn't strong enough to create a permanent bond - in fact it peels right off afterwards. But most of the time it will hold everything long enough to get a true casting.



In this picture, I've poured the second gusset piece and scraped the plaster level.



Without taking the mold apart, I pour the poll piece as soon as the gusset is dry enough for me to flip the mold over.



Here are the inner pieces after they have set up. At this point I have become rather optimistic that this mold might just work. This is what the plaster mold will look like after the first piece has been removed, and I'm impressed with how easily it pulls. Perhaps I won't need to make the second version of the mold after all!



I was also thrilled to see that my two gusset pieces separated easily, giving me a clean split.

I'm so optimistic at this point that I just barely avoid making a really, really big mistake.

A brief moment of insanity

I interrupted my moldmaking for some unplanned home renovation. We're not usually the kind of people to paint their house on a whim - and certainly not one week before Christmas. In fact, our house has been the same "contractor off-white" since we built it twelve years ago.

That's twelve years living in a place with less color than the average hospital room.



The builder said these two-story rooms would make the house "light and airy". My (tall) husband loves them. I have simply been frustrated that I had no way to paint 22ft. walls. (Light and airy contractor beige got old really fast.)



So when our neighbor told us he knew a good painting crew desperate for work thanks to Charlotte's stagnant construction industry, we asked for a quote. Two days later, my beige walls are gone, replaced by varying shades of sea greens and soft blues.

The change is worth every bit of disruption and inconvenience. But I have gotten behind on everything, including my moldmaking posts. The first mold is actually finished.



See? All done and drying. I did take pictures all along the way, but I need to get them sorted and color corrected. My plan is to do that tomorrow... after I put all the Christmas decorations back up again!

Shaping the gusset piece

Here is my first plaster piece after it has hardened. So far everything is good, with no shifting of that raised foreleg. (On small pieces, stabilizing the legs is the biggest challenge.)

I will need to shape this first piece, though.



Splitting the rubber gusset after it has been poured avoids a mold line on the rubber original, but the downside is a rough side where the two gusset pieces meet. I will have to smooth the area.



Here I've done the basic shaping for the area. What I want is a smooth transition from the edge of the leg (which I cannot disturb) to the other side. I also have to be careful not to create parts that are really thin. Plaster is a soft material, and thin or pointy areas tend to chip off once the mold is in use.



This is the other side of the front piece, and it shows why I needed to slope the cut back. If I had cut straight across from the other foreleg, I would have cut across this opposing leg. There is also a bit of my rubber sprue line visible, so I'll need to sand that smooth as well.



Here is my finished front gusset. I can already tell that this piece is going to be the evil one for this mold, because there wasn't any way to avoid some really thin, really pointy parts. But it is smooth, and I am pretty sure it will drop from the casting (rather than have to be wiggled off). I think it will work! Now all that's left is to seal the non-design areas with mold soap.



Here I've inserted the properly shaped, soap-sealed front gusset into the master mold. In the next step I'll be pouring the second gusset.



I've also taken out the poll piece so I can pour it without having to disassemble to mold. The general rules for small horses is never remove pieces you don't have to, and take apart the mold as little as you can manage. That keeps everything in proper registration.

The first plaster pour

I use #1 pottery plaster. Household plaster doesn't work for moldmaking!



And the mixing tool is even cheaper than my dollar-store spatulas! (Everything about plaster is cheaper than rubber.)



Here I just poured the plaster for the front gusset piece. The rubber version of the same piece is on the table to the left. My high-tech air bubble removal system is there in the front of the picture.



Once the plaster has set to about the consistency of paste, I scrape the piece flush with the outside of the mold. This is important because I'll have to start boxing the mold up with Legos again when I pour the large side pieces.

This will now need to sit while the plaster cures.

Preparing the master mold

I will need to do a few things with my master mold before it's ready for the plaster. The biggest of these is going to be separating the gusset into two pieces. Normally this is done after the rubber copy has been made in order to keep the mold lines on the original to a minimum.

I need to do this because the gusset piece will not release the casting without risking a break to one of the legs. The problem with Imp, however, is that those legs (and the necessary pour hole) occupy a really small space. It's going to be tight working no matter where I cut.



I decide to run my split down the forward tripoded leg. The gusset usually splits to either side of the pour hole. I've already had to make a pour hole that does not bisect the gusset because there wasn't enough room between the two legs on that side. It was tempting to split the gusset along the pour hole and spare that front leg the mold line, but I suspected that design wouldn't really give me much more wiggle to get the piece out with all its legs intact.

The downside was that it would be a complicated cut. I would need to carefully cut the line down that inner front leg, but it would need to fall quite further back if I was going to miss truncating the raised foreleg on the other side. (This will become more clear in the next post with pictures of the resulting piece.)

I should add here that I hate cutting rubber. The rubber I use is really tough. It doesn't cut easily and it will quickly dull an Xacto blade. The important initial slice down the leg, using the point of the blade, isn't hard. After that I spent most of the time thinking, "This is a recipe for lopping off a finger" and "gee, maybe I should have placed the phone next to the workbench in case I have to call for medical help". The split was rough, but I cut it without incident (other than a few new gray hairs).



With the gusset split, all that was left was to fill the sprues. I'm not really picky about getting these just right, especially since the plasteline doesn't really stick well to the rubber. Mostly I just want to cut down on the clean-up I will have to do with each plaster piece.

Now I am ready to start. The basic idea behind a rubber master mold is that, one by one, the pieces will be replaced with a plaster version, until all that is left is a plaster mold encasing the rubber original. The rubber original is then removed and the plaster mold is ready for ceramic slipcasting, and the rubber pieces can be reassembled to make the next plaster mold.



When I began the rubber mold, I started with the big side pieces first, and worked my way inside. The plaster molds go the other way; I start on the inside and work my way out. For this mold, my first piece is going to be the smaller, forward gusset section. That piece gets removed and the rest of the master mold is reassembled.



In the next step, I'm going to be pouring my plaster into the cavity left by the missing piece. The rest of my master mold is going to act as a framework with the rubber original creates the image in the plaster.