Okay, this is another topic which is really loaded with personal preference. Essentially they all accomplish the same goal, so if you find a method, which varies from mine but works for you, great.
Now I hate making molds for several reasons.....
1) They take time. I spent damn near 8 hours once making a large mold (this was mainly due to ambient temperature affecting cure time though).
2) There is great potential for screwing it up. You could very well spend 3-6 hours making a mold only to find out that some major flaw has rendered it useless (which you wont find out until the process is done and you've potentially torn apart your sculpt in the process as well). So no pressure.
In reality, it has to be done and you will get better as you make more and more molds. So without further adeiu....
Sealing your sculpt
First off, if your sculpt is made from water based clay, it's best to let it firm up a bit before attempting to mold. This usually occurs naturally since many folks sculpt in basic shape and anatomy while the clay is new and soft, and then will move on to detail after it's been worked with for a while and has firmed up a bit. Either way, you want it firm, but not dried out and cracking.
Now I didn't have a sculpt ready to take pictures of the process with, but fortunately I had to make another mold from a latex master I have. This particular master has a neck too small to use a one piece mold, so I will be doing a two piece mold which is exactly the process I would use on a sculpt as well.
The first thing I'm going to need to do is prepare my sculpt for molding. this will provide a barrier or 'release agent' between the sculpt and the mold material. This will also increase my chances of salvaging a sculpt if the mold doesn't take.
The two aerosols we are in need of are clear coat and dulling spray. Krylon makes both of these, but I can't find dulling spray in any local store to save my life, so I use some matte finish spray from the same brand of clear coat I use.
You want to apply a very light layer of the clear coat first to your sculpt. This will create a shiny surface. Then apply a thin coat of dulling spray. The dulling spray will take the sheen off of the clear coat and prevent the liquid plaster from running off the sculpt (like water off a freshly waxed car) as you attempt to apply it. I still seal my latex masters when making new molds from them. The latex can potentially absorb moisture from this master, so I still apply a super thin coat of dulling spray to it prior to molding.
Now, since you will need to get your sculpt OUT of the mold when it's done, the mold will be made in two halves. In order to accomplish this, a dividing wall must be made around the vertical plane of the sculpt. Now some folks make this from clay, others will use playing cards. I use clay myself.
By now your sculpt should be fairly firm, but you still need to take care when applying the dividing wall. If you've sealed your sculpt properly, there's no problem using some soft water based clay to make a dividing wall on a water based sculpt. They shouldn't mix and I've never had a problem with it.
I usually treat my dividing walls as part of my sculpt. The cleaner the work, the better the end result will be. Resulting pulls will only be as good as the mold they're taken from, so please take your time doing this.
Here's some stuff you will need:
Now the wooden board is something I made for cutting clean slabs of clay. It was from the leftover MDF I had from the lazy susan.
Now, it's best to follow the contour of the ears when making the wall, it's easier to get it flush with the sculpt, and easier to clean the seam when the resulting masks are done. You don't want a line running down the middle of the ear from where the mold separates. I usually make my dividing wall about 2.5 to 3 inches high and about 1/2 inch thick.
Now, notice the gap between the sculpt and dividing wall. This is a royal pain to get a nice flush dividing wall with a latex master that wants to flex, but you'll face similar issues with your sculpt. You really want to get a nice clean transition between the sculpt and dividing wall. This will prevent any plaster from getting under the dividing wall and will provide a base for the second half that will result in a nice tight seal.
Use your mad sculpting skills to get this as clean as possible. You can use a wet paintbrush to seal the gap since it will smooth the dividing wall clay well. Just try not to run it off on your sculpt.
Now if you've ever seen a sculpt with a dividing wall, you've usually seen small round hemispheres used as mold 'keys'. These help line up the mold and I've used them on larger sculpts, but this one is small and the area around the ears where the wall flexes out will provide the keying for me. I did scribe small valleys with my sculpting tool on the top and under the ears and these serve two purposes. They provide additional keying, and will help prevent latex that might leak between the two mold halves from leaking out of the mold. You really want some sort of keying though to get the mold to line up right when you join the two halves.
Not my best work, but I don't have all day to pretty it up any more. When you get it nice and clean, now comes the fun part....
Mixing and applying plaster
Okay, two of the popular plaster types are Ultracal and Hydrocal. They're both gypsum products and provide a nice hard detailed medium for molds. Personally, I've only used Ultracal 30, but plaster is pretty universally mixed and applied. Curing times and ratios may vary, so always consult the proper documentation for your particular product.
Ultracal 30 comes in 50lb bags. I always transfer mine to a 5gal bucket for ease of storage and use. Also, during the molding process, it is very handy to have a 5 gal bucket with a few gallons of water in it ready for cleaning your hands, tools, and such.
Pouring unused plaster in liquid form down the drain is a sure fire way to ruin your plumbing. I always try to use ALL the plaster I mix and clean my buckets and tools by heavily diluting it first. You can always let it cure and break it off and throw it away.
Okay, some materials you might need:
1) Plaster, of course
2) Mixing buckets. I have my buckets shown that I use for resins and such, but any bucket will work, preferably 2-5 gal depending on how much you're mixing.
3) Chip brushes, for applying the plaster.
4) Latex gloves for application(optional). I don't use them myself.
5) Spatula. This is great for getting all the plaster in the bucket gathered up, but I don't use it for mixing.
6) Mud knife (optional). I use this to clean up any splashed or dropped plaster around the sculpt. I try to clean as I apply layers so I don't have a huge mess at the end. It also makes for a nice clean edge on the bottom of your mold.
7) Burlap strips of various sizes. This is for reinforcing the mold (I have never had a mold break on me)
Ultracal 30 has a mixture ratio of 3:1. This means that for every cup, ounce, or whatever of water, you add 3 cups,ounces, or whatever of plaster. This is great and all, but since powder can be measured dense or loose, it's not a very accurate way to mix. You really want to use it as a baseline and try to rely on consistency.
Now, you want to start off with your water in your bucket and add the plaster slowly. I sift it with my hand allowing it to absorb the water. If you dump it in, you'll get some real messy results.
They say the trick is to keep adding it slowly until it starts to form islands on the top and appears as a dry river lake bed. I find this is sometimes not enough, but you want at least this much.
Now I don't suggest using any high speed agitation or wisk for mixing since this could potentially introduce air into the mixture. I mix it up with my bare hands since it gives me a good feel for the consistancy.
The first batch you want to mix up will be the thinnest mixture. This is called the 'splash coat' and is one of the most critical because:
A) It's where all the detail of your sculpt will be picked up
B) any unwanted air bubbles will be noticed on your masks.
I like to equate the splash coat consistency to buttermilk or thin pancake batter. Now this stuff sets up fairly quickly, so you don't want to go goofing around. Start applying this from the bottom up with your chip brush.
Now, you'll notice some thin spots, I went back to those with the remaining plaster in the bucket.
You'll mix up seperate batches for each coat. For the 2nd,3rd, and even fourth coats, you want to be careful and avoid using brush strokes since this could tear apart the thin splash coat layer. simply apply it lightly with the brush. If you throw on too much too fast, the underlying layer could crack or slide. These first few layers are critical and you want each to set completely before applying the next layers.
Once you have a good few layers, it's time to lay on a thicker layer. I mix up a good batch a bit thicker than the previous and simply slap it on, you're not going for detail here and simply building up the mold now.
Once you have a good foundation, it's time to add a layer of burlap. Some say this is overkill, but doing this right will ensure a strong mold since the burlap absorbs a lot of plaster and it a great reinforcer.
Mix up a nice thick batch of plaster, dip a strip of burlap in it completely and apply it to the mold. then add another stirp overlapping the edges by about 1/4 of an inch. do this until you have completely covered the mold. Use any extra plaster to apply a layer on top of the burlap.
(Notice the splashing I dripped near the base of the mold, keep tidying that up with the mud knife)
Now keep building this up until you have a nice thick mold (aprox 1.5 to 2.5 inches). You can check by looking at the profile and from above to see how much you've built up as pictured.
Now comes the smoothing. This is also optional, but since you will be handling this block of stone, you want it to be as smooth as it can be. Skipping this step can result in rigid molds that can have sharp egdes. The trick here is timing. You want to catch the last layer of plaster just as it almost fully cured. Wet your hands and try smoothing out the surface. If yo're sloshing around the final layer, it's too soon. You can do this in two steps. Rough smoothing and final smoothing. It's something you just have to do to get the feel.
This is somewhat how the resulting half mold should look.
Molding the front
Now we need to prepare for molding the second half. you need to remove all the dividing wall clay carefully. This will hopefully leave the clean exposed edge of the back half of the mold. Use wooden tools and a little water to remove all the clay from the mold and anything that may be stuck to your sculpt.
Now we will add separation keys. These will help us pry the mold apart once it's done. I place two towards the bottom and two towards the top:
You want these placed at least 1/2" away from your sculpt, otherwise they will provide a channel for you mold to leak excessively. I make them about 1/2" thick and 3/4" wide. Smooth the transition between them and the mold with a wet paintbrush as you did the dividing wall.
Once you've got your keys made, you need to add a release agent to the exposed mold where the dividing wall was. If we simply molded the front, it would fuse to the back half defeating the purpose of the dividing wall and forever encasing your sculpt in a cement tomb.
Best thing for this is vaseline (or any generic petroleum jelly). This is where the gloves come in handy. Apply a thin layer over the ENTIRE surface of the exposed mold wall AND about 2 inches over the back half of the mold. You're basically preventing any plaster that hits the wall or spills to the back half from fusing.
Now, for the area where the mold meets the sculpt, I am a bit more critical since I don't want a huge buildup of grease preventing a good seal. I take an empty soda can turn it upside down on the concrete. Put about a tablespoon of vaseline in the concave cavity and hit it with either a torch (from a distance), heat gun, or hair dryer. This will liquify it temporarily and provide me a brushable liquid to create a thin seal between the mold and sculpt.
I go through great lengths to ensure that there are no unprotected areas of plaster from the back mold to fuse with the front. Taking your time will reward you with a mold that is easy to separate, and one that seals well.
For the second half of the mold, we wont have to fight gravity as much. You can lay your sculpt carefully on the back and mold the front half exactly as you did the back, burlap layer and all. Try to avoid letting plaster creep down the back half of the mold, and clean up any that does.
Once you finish molding and smoothing the front, tilt the mold upright, and dig out the clay that you made the separation keys with. Grab yourself a screwdriver or small prybar and carefully start to separate the mold. I usually do a little on one side and then a little on the other. Keep alternating until one half of the mold is free.
Since the face holds all the detail, the back half is usually the side to come off first.
Once removed, clean off all that petroleum jelly and any clay that may still be stuck in the mold. I've taken molds to the carwash and blasted them a bit with the pressure washer (from a distance) to clean out the clay as well. A spongebath on the inside can also remove any residual clear coat that may have stuck.
Now if there are small pockets of air in your mold, don't fret. You can often repair them with a small batch of plaster as long as you keep it flush with the surface of the mold. any buildup of plaster on the inside of the mold will create indentations on any resulting masks. I've also use extra clay to fill in small holes and inperfections.
I would HIGHLY recommend taking some clay and making a small face, from the ears forward, on a board. Then making a small mold of it. This will give you a feeling of working with the plaster and the resulting half mold. It's good practice before moving on to your actual sculpt.
Next installment, casting a latex mask.................