A Guide for Mixing Pottery Plaster
It seems pretty simple: combine wet and dry ingredients, stir it a bit, then dump the resulting mixture into another container.
You could say the same about baking, but Sharie will attest to my unfortunate luck with cookies and cakes! There's some tip, or trick, or 'life-hack' to it that I just can't seem to hold onto, and most things come out a little bit flatter than they should.
And just as in baking, mixing up a batch of pottery plaster is a process with lots of potential hurdles balanced by lots of simple tricks that, once you know them, will help you turn out a great product time and time again.
I have made some mediocre molds. Some of them I managed to use for, perhaps, an embarrassingly long time. Because I was so intimidated by the whole mold making process, I was willing to clean tiny bubbles from every surface of every casting from those imperfect molds, even as they were getting worse with each use.
Here, I'd like to prevent that from happening to anyone else. Most of my knowledge comes from a mixture of research, trial and error, collaboration with others, and experimentation. Through all of that I have developed a process that produces a consistent plaster for creating durable molds without surface bubbles or weak spots.
So, read the entire post to learn about how I mix up a batch of this magical gypsum dust for making slip casting molds. Then snag a copy of the printable 'recipe' at the bottom to keep all the steps in order without fuss!
Gypsum plasters are surprisingly common. The most famous, and most base version is 'Plaster of Paris". This plaster, known by its source: a literal hill of gypsum near Paris, is a powdered form of calcined gypsum.
Said another way: rocks from a hill in France are heated (to hilariously low temperatures of up to 356F°) then ground into a fine white powder which has the surprising gift of recrystallizing back into rock.
'Pottery plaster' is a specific gypsum plaster with special chemical additions to achieve ideal material characteristics for use in the ceramics industry. There are a surprising number of plaster products available, and it is important to make sure that you have the type you need.
USG No.1 Pottery Plaster is what I mix for all of my slip casting needs. I have also used it to make master molds, plaster bats, and in life casting. Don't be fooled by No. 1 Moulding plaster, as I was in the early days... All of the USG product bags look very similar, so be sure that the label on your bag matches the use case in your heart.
With the right plaster and some water, you can make a mold (or whatever). So why bother with this guide at all?
Pinholes at or near the surface, bubbles formed in tight areas, weak spots, inconsistent absorption, plaster that binds up, plaster that doesn't set up, plaster that doesn't absorb water, and plaster that doesn't hold detail are all
possibilities when mixing up these liquid rocks. Also, pouring plaster is often the last step in a very long series of events that, due to its’ rigid and binding personality type, has the potential to ruin a project.
Sometimes, plaster can 'destroy' your model even if it leaves a totally usable mold.
With all of that being true, a clear set of instructions sure wouldn't go astray...
If you consult the manufacturer's website, you'll find an array of documents about gypsum cements. The one specifically about this particular powder contains helpful, if surprisingly non-specific, information. The Submittal Sheet leaves out a lot of hard numbers, opting for a not so pushy set of instructions that could use a small dose of demystification.
Each step of this process has room for adjustment to your particular needs. Keep in mind that whether it's achieved by fortifying the plaster content, or through extra vigorous mechanical mixing, a 'stronger' mix will provide poorer absorption. Also, having a multi-part mold with different strengths of plaster can result in castings that tear at the seams, or dry unevenly and crack. Sticking to a recipe ensures that each part of your mold will have very similar levels of thirstiness.
A mixture created with the steps outlined in this guide creates a plaster mixture that makes uniform molds for use in slip casting. If mixed as indicated, every batch mixed will consistently be absorbent and durable enough to perform well through many castings.
It's easy to overlook this part at first, or to assume it's way too early to consider, but preparing for the mess that is 'plaster' lets you focus on the important stuff. Since our plaster starts hardening within 8 minutes of contact with water, and we use 4 or more minutes to mix it properly, the moments saved by organization are curial.
It's also extremely important to consider where any excess cement might end up. It's usually worth making a little extra, since having too much is better than the (extremely disappointing) alternative, but where do you dump any leftovers?
It might be obvious, but it has to be said anyway: Never pour gypsum cement down a drain! Mixed gypsum cement is like any cement and will harden in your pipes and clog them. Powdered gypsum plaster will mix with the water in your pipes and will harden, causing a blockage. Plaster bits are heavy and will sink to the bottom of ‘p-traps’, eventually stopping water from flowing through them.
No matter how much extra rinsing or diluting is done, gypsum cement should never go down a drain.
This is where the 'Cleanup Bucket' comes in. Unless you are using a lot of disposable kit, this humble tub of water will make the whole process an order of magnitude easier. Any bucket large enough to easily clean your mixing container will do, just fill it with enough water to submerge your tools and mixing bucket, then have it close enough to access without tripping over it. When you are finished mixing your plaster, simply swish your tools around in the water, and voila, they're clean! Once your mixed plaster is gone (excess mix can be poured into a trash bag, empty plaster bag or any disposable container), do the same for the mixing container!
It really is that easy and will save hours of chipping plaster off tools and out of mixing containers.
Over time, the gypsum will settle to the bottom and the clear water can be poured or siphoned away, and anything remaining can be unceremoniously dumped in the garbage. Alternatively, I line a 5-gallon bucket that has 1/2" holes drilled in it for drainage with an empty plaster bag and pour the whole mix in to let it filter itself out. If you use this second method, be sure to consider where this not-quite-potable water will be seeping to.
Even if you're only working with this stuff occasionally, it's worth thinking about the effects it might have on your health and wardrobe!
Remember how gypsum cement is just warm, ground up rocks that turns back into rock? Well, it doesn't differentiate between a bucket of water, a drainpipe, or even your airways. Plaster powder is very fine and creates quite a few airborne particles, and since lungs are just a network of wet tubes, it is definitely worth your while to equip yourself with some sort of respiratory guard. I recommend a simple N95 mask, but even a dust mask or handkerchief to cover your nose and mouth is great!
Plaster is also hydrophilic: it loves and needs water. If that water is bound up in the skin of your fingers, that's your problem. Personally, I always wear long wristed cleaning gloves when dealing with this very dry powder, and even prefer to use gloves if I'm just going to be handling a lot of plaster molds.
A constant 'splash zone' exists when mixing up a batch of plaster, this puts both clothes and eyes in danger of a direct hit. While it is not entirely impossible to remove from fabric, it's better to avoid stains in the first place. Wear an apron over clothes that you don't mind getting dirty in the first place for minimal risk.
Of course, the amount of personal safety equipment you choose to use is entirely up to you and what suits your needs.
After years of constantly working with so many different types of dust. I've learned that this kind is well worth taking the extra time and care to avoid. 🙂
MISE EN PLACE
Usually a phrase reserved for kitchens 'mise en place' simply implies having everything in its place, ready for the moment it will be needed. This process greatly benefits from using the same ideology, which is why I set up my workspace completely before weighing my first gram of water.
There are three areas to have ready, ideally within one step of each other forming a sort of triangle shaped workflow. One area for mixing, one containing the cleanup, and the last area is the pouring station.
- Here's how I lay everything out:
- Pouring area
- On a very flat, very level, very stable surface
- Protected from incidental splashes during mixing
- Any release agents / lubricants applied
- Watertight containment field
- Isopropyl Alcohol
- Towel (scraps/rags)
- Cleaning area
- Lined trashcan or disposable container for excess plaster
- Prepared cleaning bucket
- Mixing area
- Water (at room temperature)
- No.1 Pottery Plaster
- WET only mixing container
- DRY only container
- Digital timer (set for 2 minutes)
- rubber spatula/spoon/scraper
- drill w/ mixer attachment
With everything laid out this way, the 'mixing' part can lead straight into the 'using' part, and the 'cleaning' part doesn't bog everything down before you can enjoy your success! This also lets you focus on the mixing process itself. Since timing is an important factor here, getting distracted could lead to wasted materials or a damaged model.
MIX IT UP
Once you have the dry and wet parts weighed out separately, it's time to strew. Likely, you strew all the time, even if that word has never entered your world. If you think about how you might salt your food, you'll be on the right track. A sifter would work great here too; getting the plaster powder as evenly distributed and dispersed as possible is the idea.
Like any cement, the plaster used here only has a certain amount of workable time. You're likely to notice early on that almost half of that time is used by the mixing process, and that half of that time is used by... not touching it. This important step goes against intuition but, much like a famous cleaning product, it does the work, so you don't have to.
Despite being powdered rocks, plaster dust floats around in the air and gets everywhere! That's because each minute particle of mineral can develop its' very own minute envelope of air, acting like a little kite that lets it fly. Each tiny 'air envelope' is insignificant, but if they congregate (like they want to in a viscous liquid), they'll form bubbles. Sifting or strewing eliminates a lot of that incidental air, while soaking allows more to be passively forced out. Stirring or swirling the mixture around at this point will tempt you. Beware that after a certain amount of agitation, the setting process really gets going and may bind up at an inopportune time (such as before or during the pour).
A modern drill at top speed will add tons of air and create a very strong (and less absorbent) end product, as well as a giant mess. If you're using a drill, set it in reverse. If it has multiple gears, or speeds, set it to the lowest available before mixing the plaster. You will mix for about 2 minutes. Be sure to scrape the bottom and edges at least once while mixing. Dragging the drill across the bottom and sides doesn't do it, I've tried! Instead, use a rubber spatula or similar. After mixing, give the drill attachment a quick spin in the clean up bucket.
To get smooth surfaces on every face of my mold I rely on alcohol and ‘surfactant’. These two chemicals do the same thing in different ways. Each can reduce surface tension and prevent bubbles from appearing on the mold surfaces.
In the case of alcohol, it reduces the surface tension of the liquid mixture creating a liquid surface so thin that even the tiniest of bubbles can't be supported.
The surfactant in this case is a mixture of equal parts windex and water. This solution works to lower the surface tension on the solid surfaces that the plaster will contact. Like water, our plaster mixture will 'bead' on waterproof surfaces, and the plaster is heavy enough to entrap air when those beads meet. Surfactant applied to a waterproof surface prevents beading, which prevents bubbles from forming on the surface of your mold.
These two liquids are used in the last moments of the process. Sitting too long can dry it out or allow it to pool, which can create weak points in your mold. I usually apply a generous amount of surfactant across the entire model just before pouring (better yet, have someone do it for you during the last 15 seconds of mixing). I then spray the wet plaster mixture with alcohol while agitating the sides of the container just before pouring, and once again when I've finished pouring the mixture into its' final shape.
Pouring should be a smooth consistent motion with no splashing or sputtering. Aim for the deepest area of the mold to start and allow the mixture to self-level over the details of your model. (Advance Advice: Pouring at the exact same spot may cause a hard spot that is less absorbent, if you have space, move about an inch throughout the pour.) Give the edges a little tap and jiggle at the end to help get a smooth, level surface and spritz with alcohol to get rid of any bubbles that come to the surface.
Once you've completed your mixing and pouring dispose of any excess plaster and use your dirty tools, gloved hands and prepared cleanup bucket to give everything a good rinse. Once your tools are dry you can put them back where they belong. I keep a plaster set all stacked together, ready to measure, mix and clean, time and time again!
Hopefully all that typing I did will be put to good use. Once you check out this great one-page 'How To' guide, you’ll probably decide you'd like some more in-depth info on this part of the mold making process. In the meantime, here is an easy to read 'recipe':
If you read this far, thanks! I hope you found some useful information or a bit of entertainment. If you did, feel free to let us know down below in the comments, on Instagram, Facebook, or through email, firstname.lastname@example.org!
I am not supported by, nor do I represent in any way, the manufacturer of this indispensable product. This guide was written and published by me using the sum of my knowledge. Part of that knowledge comes from reading information provided by USG; this link takes you to the No.1 Pottery Plaster informational pages provided by them. I use this chart as a quick reference for water to plaster ratios.
Another notable font of information came in the form of Youtube videos posted by VanTiki, an incredibly talented sculptor, ceramicist, and sharer of knowledge check him out for all kinds of interesting content!