A Clay Reclaim Process Using a Pugmill/Clay Mixer
High Production = Lots of Clay to Recycle
If you run a community studio or your personal studio is high production, you probably have a lot of clay scraps to deal with. This article will describe the clay reclaim process we use at the very busy pottery studio where I worked as technician for 6 years.
It’s not only important to get the clay back into a good working state with regards to water content, but it’s just as important that the recycled clay retains its plasticity or you’ll end up with short clay that cracks easily, resulting in a lot of failed pieces and unhappy potters.
When you know what to add to your clay reclaim to increase its plasticity, you can produce recycled clay that’s just as good, if not better than fresh clay out of the bag. You just need to add the right ratio of clay particles.
If you don’t have a pug mill/mixer, I have A Low Tech System for Recycling Clay Scraps that doesn’t require any fancy equipment and doesn’t take up a lot of space that’s perfect for small studios.
If you do have a pug mill/clay mixer, or you’re thinking of getting one, this article is for you!
(Note: I use the words “reclaim” and “recycle” interchangeably when it comes to clay. I also use the terms “mixer”, “pugger” and “pug mill” interchangeably.)
The Pug Mill/Clay Mixer
The first thing to note is that not all pug mills are created equal. They can either be built for mixing large volumes of clay, or for de-airing clay (no wedging required), or both.
The pug mill we have is called a Peter Pugger and it’s a clay mixer only. It doesn’t have a vacuum and the clay “pugs” that are produced still need to be manually wedged to remove air pockets.
Our Peter Pugger can mix up to 250 lbs of clay at a time. During our busiest summer months where we offer kid’s clay camps, there’s a lot of clay that ends up in the clay recycling bin and we can sometimes process over 500 lbs of clay per week!
That’s a lot of clay that gets used and reused and the more clay gets handled, sponged, wet, dried, rinsed down the drain etc., the more chance you have of ending up with “short” clay that has lost its plasticity.
Plasticity vs “Short”
Plasticity refers to a clay body’s ability to be formed into a shape, and to retain that shape. Clay is what’s called a “plastic” material.
Short is the word that’s used to describe clay that has low plasticity, where clay is prone to cracking when bending, stretching and shaping.
This is short clay on the wheel. The edges are tearing from being stretched.
If you roll a coil of clay and then bend it, it should bend nicely and not crack. If it does crack, it’s considered short.
5 different porcelain clay bodies, ranging from plastic on the left to short on the right
Same goes for cutting a thick slab from a block of clay. If you bend it and stretch it, it shouldn’t crack like this clay did. This clay is short.
I know, it can be confusing to refer to clay as “plastic” when your brain automatically thinks of something completely unrelated like plastic water bottles. But we want our clay to be plastic, or have good plasticity.
Plasticity is not to be confused with “elasticity.” Elasticity refers to a material’s ability to be stretched or deformed and then return to its original shape. Plasticity means it can be molded into a new shape and it retains that shape.
A clay body that has sufficient plasticity will not crack when it is bent or stretched.
How the Plasticity of a Clay Body Works
Wet clay particles attract each other like magnets. Water allows the particles to move and slide and stay stuck together as we shape the clay.
The other thing that’s required for plasticity is what’s called “clay particle size distribution”.
The better the distribution is of small, medium and large clay particles, the more points of contact we have between all the particles, and the better our clay will stay stuck together.
Here are some graphics to help explain what I mean. Note that clay particles are flat and hexagonal, not round, but you get the idea.
Large and medium particles
Large, medium and small particles
When we only have one size of clay particle, there are a lot of gaps in between them and less overall points of contact bonding the clay body together.
If we bend or stretch a clay body containing only one particle size, because there are fewer bonds, a lot of those bonds will break, causing the clay to crack. This is what happens when clay is short.
When we have a mixture of particle sizes, the small and medium particles fill in all the gaps between the large particles and now we have more points of contact holding the clay body together.
Remember, wet clay particles attract each other like magnets so the more points of contact, the stronger the overall bond throughout that clay body will be.
When clay is short, it means there aren’t enough points of contact, holding the clay together. There are gaps that show up as cracks in the clay.
Plasticity and Clay Recycling
Ideally, when working with clay we save all of our clay scraps, including the clay in the throwing bucket, the splash pans, the sponging water and the clay we rinse off our hands.
The clay particles that end up in the various buckets of water around the studio are very important contributors to the plasticity of our clay body. These are generally
the smaller clay particles and if we discard them, we’re left with gaps in between the large clay particles. Those gaps show up as cracks in our short, recycled clay body.
If we’re the only one handling our clay, we can make sure we save all the clay particles and our reclaim is often great to work with. But when you work in a public pottery studio, you can’t control how much clay ends up in the recycle bin and how much ends up down the drain (and into the clay trap, of course).
To compensate for this loss of clay particles we can add them back in when we put clay into the mixer/pugger. We have a dry clay recipe that includes small, medium and large clay particles that we add to the pug mill every time we pug a new load. Recipe is at the end of this post.
The dry clay not only helps with plasticity, but it helps to dry out the soggy wet clay that ends up in the clay recycle bin so there’s no need to spread it over a plaster batt. We bring it to a wedging consistency right inside the pug mill and it’s ready to use as soon as it comes out.
We bag the pugs and label them with the date so we can ensure “first in, first out.” This gives freshly pugged clay some time to rest and prevents older clay from drying out.
A Community Studio Clay Recycling System
1. Collect all clay from studio users
We have a large plastic bin on wheels (30”L x 19”W x 18”H) that studio users put all their clay scraps into – wet, dry, leather hard, everything. The stage of dryness doesn’t matter, but no bisque. We want as much clay as possible to get into the bin before they start rinsing their tools etc. in the sink.
Our sinks have traps so clay can be safely rinsed down the drain, but we encourage as much recycling as possible.
2. Pull out dry/hard pieces of clay so they can dry out completely
Before we transfer clay from the large recycle bin to the pug mill, technicians/volunteers pull out any leatherhard to dry clay and put it into a separate bucket. The criteria for removal is any piece of clay that breaks instead of bending.
The reason we remove the drier clay is because our turn around is very fast and we don’t want lumpy clay with hard bits mixed in. If we were to leave all the dry and wet clay together for long enough, the dry clay would eventually absorb enough moisture to soften, but we don’t have time for that.
The reason we don’t have a separate bin for dry clay in the main studio is because we don’t want to expose studio users to buckets of dry clay that create a dust hazard. These buckets live in our clay room that only technicians/volunteers have access to.
3. Slake down the bone dry clay
We let those almost dry pieces dry completely. Once they’re bone dry, we fill the bucket with water and let it sit until it has slaked down. Usually a day.
Bone dry clay is porous and will absorb water easily and turn to wet clay slip. Once the bucket has slaked down, clean water is poured off the top and the wet clay slip is dumped into the clay recycle bin.
4. Fill the pugger about 75% full and mix
All soft/wet clay is transferred into the pugger by hand. We manually squish check all the clay as it enters the pugger so we can make sure random tools are left out.
We only fill the mixing chamber about 75% full so there’s room to add the dry clay.
We turn on the pug mill and allow it to mix for a few seconds. When we open it up, there’s often more room and we continue to fill if it’s not yet 75% full.
5. Add dry clay to mixer
We then add dry clay to the mixing chamber. This helps to both dry out the wet clay and increases the clay’s plasticity.
We add the dry clay to the mixer proportionately by the scoop (while wearing our fit-tested respirator). Scoop size doesn’t matter as long as you’re using the same scoop for each material. You’ll figure out the best scoop size for your pugger.
The important part is getting the correct proportions of clay particle sizes for plasticity. The number of full recipes we add is determined by how wet the clay is in the pugger.
We add one full recipe of dry, then let it mix for half an hour or so and then check to see how wet the clay is. Keep adding a full recipe and then mixing until clay is wedging consistency and no longer sticky.
Test the clay by rolling a coil between your palms and wrapping around your finger. If it’s short, it will crack. It will also crack if it’s too dry so if you suspect it’s too dry, just add some water and mix some more.
6. Pug the clay
Once the clay no longer sticks to our hands and can successfully be rolled into a coil and bent without cracking, it’s ready to be pugged. We bag ours up because they’re easier to store and move around between rooms in the building. We reuse the bags as much as possible.
We also sell the pugged clay at a reduced price to our studio users. Each pug is about 10 lbs of clay.
Other studios I’ve heard of fill a large plastic bin with pugs and keep the lid on so they don’t dry out. This saves a lot of soft plastic.
Dry Clay Recipe
This recipe makes a buff, cone 6 stoneware clay body (similar to Plainsman M340).
- 2 scoops of ball clay (medium particles)
- 1 scoop of fire clay (large particles)
- 1/2 scoop of EPK (small particles)
- 1/4 scoop of Custer feldspar (flux for vitrification)
- 1/8 scoop Frit 3124 (flux and boron for vitrification at cone 6 – omit for high temp)
- splash of vinegar (for good luck!)
I hope this article gives you a good recycling system for your high production studio so your clay reclaim is always great quality and nice to work with.
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