Interesting Clay and Glaze Additives
Grog, sand, paper, nylon fiber, sawdust, silicon carbide. Things people mix into clay and glaze to change how they behave, and a fair number of things people try because somebody on the internet did. Here is what each one actually does, which are worth your time, and which should never go in a shared studio.
Grog and sand open a clay body up and stop big work cracking. Paper fiber makes clay behave like nothing else and lets you join dry to dry. Most other additives are either niche or a bad idea in a shared kiln.
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Almost everything is about drying
Clay cracks because the outside dries and shrinks while the inside is still wet and full size. The bigger and thicker the piece, the worse that fight gets.
Nearly every useful additive is a way of letting water out more evenly. Grog and sand open channels through the body. Paper fiber does the same and leaves reinforcement behind while it does it.
The rest either change the fired surface, or they are somebody's experiment that worked once and got repeated.
Which are worth it
- Grog. Worth it, and usually easier to buy already in the clay. Big work, sculpture, slabs, anything thick.
- Sand. Cheaper than grog and rougher on your hands. Same job, coarser result.
- Paper fiber. Genuinely worth learning. Light, strong while green, and you can join bone dry to bone dry, which nothing else allows.
- Nylon fiber. Useful for green strength on thin or fragile work. Burns out completely.
- Sawdust and organics. Burn out and leave a pitted, porous surface. Decorative only, and they smoke.
- Silicon carbide in glaze. Makes craters and foam. Spectacular, difficult, and it fumes in a way that affects the whole load.
Anything that burns, smokes or fumes needs clearing before it goes in a shared kiln.
Three things that go wrong
It went into the reclaim bucket
Grogged clay blended into a smooth body makes everybody's clay gritty, permanently. Paper clay in a bucket rots. Keep anything you have modified out of communal reclaim.
It bloated or blistered in the kiln
Organics that had not fully burned out before the clay sealed over. Thick work with sawdust or heavy paper content needs a slower bisque, not a normal one.
Your hands are wrecked
Sand and coarse grog abrade skin all session. It is the reason people who throw heavily grogged clay every day end up with split fingertips, and the reason to keep grog for hand building.
Why add anything at all
Commercial clay is already a blend somebody worked out carefully. Adding to it is worth doing for a handful of specific reasons and not much else.
- To stop large work cracking. The main reason, and the one that actually justifies itself. Opening a body up lets water escape from the middle instead of tearing the piece apart.
- To hold a shape. Grogged clay stands up better while wet, which matters for slabs, coils and anything built tall before it has stiffened.
- To survive thermal shock. An open body copes better with rapid heating and cooling, which is why raku and flameware bodies are heavily grogged.
- To get a texture. A gritty surface, a toothy edge, sand showing through a thin glaze. Sometimes the whole point.
- To make something impossible possible. Paper clay is the real example. It lets you join dry to dry and repair a bone-dry crack, which ordinary clay simply does not do.
- To change a fired surface. Burnout materials for pitting, silicon carbide for craters. Decorative effects rather than working improvements.
The reason not to
Every addition changes shrinkage, and shrinkage is what glaze fit depends on. A glaze that fitted your clay may craze on a modified version of it. Worth testing on a tile before it goes on a piece you care about.
Grog, sand and the openers
These all do the same job by slightly different means: put hard particles through the body so water and stress have somewhere to go.
Grog
Fired clay, crushed and graded. It has already shrunk everything it is going to shrink, so it holds the body open while the clay around it contracts. Sold by mesh size: fine for throwing bodies, coarse for sculpture. Usually 10 to 25 percent.
Sand
Cheaper, easier to find, and sharper. It works, and because it is silica rather than fired clay it behaves slightly differently in the melt. It is also noticeably harder on your hands than grog of the same grade.
Molochite
Grog made from white china clay rather than ordinary fired clay. Same job without the color, which is why it is the one used in porcelain and white bodies.
What you gain
Less cracking, less warping, better resistance to thermal shock, and a body that holds its shape wet. On anything large or thick, this is the difference between a piece surviving and not.
What you give up
Smoothness. Grogged clay is rough to throw, abrasive on your fingers over a long session, and it will not take fine detail or a burnished finish.
The easier route
Buy a clay that already has it. Blending grog in by hand is real work, and a commercial grogged body is mixed far more evenly than you will manage on a wedging table.
Paper clay, the strange one
Cellulose fiber mixed into clay slip. It behaves so differently from ordinary clay that it is almost a separate material, and it does one thing nothing else can.
- You can join dry to dry. The headline. Score, wet with paper slip, and a bone-dry piece bonds to another bone-dry piece. Ordinary clay cannot do this at all, and it changes how you can build.
- Enormous green strength. The fiber acts as reinforcement, so thin walls and long extensions survive handling that would snap normal clay.
- It dries far more evenly. The fibers wick water out from the middle, which means less cracking and less careful drying.
- It is lighter, and it stays lighter. The paper burns away and leaves voids, so a fired paper clay piece weighs less than the same form in ordinary clay.
- You can repair cracks in dry work. Paper slip into a crack on a leather hard or bone dry piece, and it holds. This alone rescues pieces that would otherwise go in the reclaim.
- The trade-offs. It is slightly weaker fired, because those voids stay. It throws poorly. It rots if you keep it wet too long, and that smell is worse than ordinary reclaim.
Making it
Toilet paper or shredded newsprint, soaked and blended to a pulp, then mixed into clay slip. Roughly 10 to 20 percent pulp by volume for general work. Make it in small batches and use it, because a bucket of wet paper clay goes bad quickly.
Fibers, burnouts and curiosities
Nylon fiber
Chopped fibers, a fraction of a percent by weight. Huge green strength with none of the texture change of grog, and it burns away cleanly. Good for thin, fragile or extended forms.
Sawdust and organics
Burn out and leave a pitted, porous surface. Coffee grounds, rice husk, seeds. Decorative only: the fired result is weak and porous, and the bisque smokes noticeably.
Perlite and vermiculite
Used for lightweight, insulating bodies. They give a coarse, open texture and a piece that weighs very little. Not a functional-ware material.
Bentonite
A tiny amount, 2 percent or less, makes a short clay more plastic and keeps a glaze in suspension. One of the few additives that is genuinely standard rather than experimental.
Silicon carbide
In a glaze, it reacts and gasses at temperature and produces craters and foam. Striking results, and it fumes in a way that affects the whole load rather than just your pot.
Barium carbonate
A very small addition stops soluble salts scumming a clay surface. It is also genuinely toxic in raw form and belongs with the materials you handle by the label.
Additives in glaze
Different job entirely. Most of these never reach the kiln; they exist to make the glaze behave between the bucket and the pot.
- Bentonite, for suspension. Two percent or less stops a glaze settling into a solid layer at the bottom of the bucket. The most useful thing on this list.
- CMC gum, for brushing and for grip. It slows drying, improves brush flow, and makes a glaze stick to surfaces it otherwise slides off, which is why it matters for reglazing a fired pot.
- Epsom salts, for flocculation. A pinch turns a watery, settling glaze into one that hangs together and coats evenly. It is the standard fix for a bucket that separates.
- Darvan or sodium silicate, the opposite. Deflocculants, which let you carry more solid with less water. Mostly a casting slip concern.
- Glass frits, which are structural rather than additives. Pre-melted glass that delivers boron. Worth knowing because they are the reason mid-fire glazes work at all.
- Everything organic burns out. Gums, suspenders and binders leave nothing behind. They change how the glaze goes on, never how it fires.
What not to put in a shared kiln
Most of this page is about your own pot. This section is about everybody else's, which is why it shows in both versions.
- Anything that fumes. Silicon carbide and the volatile colorants travel through a kiln and land on other people's work. Clear it first.
- Heavy organic burnout in thick work. Sawdust-loaded clay needs a slower bisque than a normal load gets, and pushed through a standard schedule it bloats.
- Anything you have not told anybody about. A modified body is not a problem; an unannounced one is, because it changes where a piece should be loaded.
- Modified clay in the communal reclaim. Grog turns a smooth body gritty for everybody, and paper clay rots in a bucket. Keep your own scraps separate.
- Found materials you cannot identify. Beach sand carries salts, garden soil carries whatever is in it, and neither is worth a damaged shelf or a ruined load.
The one thing that makes all of this fine
Tell somebody. Almost none of these are banned, and almost all of them need to be loaded differently or fired more slowly. A sentence when you hand the piece in is the whole protocol.
Mixing it in
- Respirator on before anything dry is opened. Grog, sand and powdered additives are exactly the dust the mask exists for.
- Work out the percentage by dry weight, not by eye. Ten percent grog means ten percent, and guessing gives you a different clay every batch.
- Spread the clay out flat, sprinkle the additive across it, then fold and cut.
- Slam wedge it in. Fifteen to twenty cuts. Each cut doubles the layers, which is exactly the tool for distributing something evenly.
- Check a cut face. Streaks or clumps mean keep going. This is the only way to see whether it is actually mixed.
- Test a tile before a pot. Fire it with the glaze you intend to use, because you have changed the shrinkage and the glaze fit may have changed with it.
- Label whatever you keep. A bag of modified clay that nobody can identify ends up in the reclaim, which is how it becomes everybody's problem.
What goes wrong
The glaze crazed when it never used to
You changed the shrinkage of the body, and glaze fit depends on it. Test tiles in the modified clay before committing a piece.
Bloating and blisters in the clay
Organics that had not burned out before the body sealed. Thicker work and heavier organic content both need a slower bisque.
Clumps of grog in one place
Under-wedged. Weak spots where the clay is mostly filler, and they show as tears when you pull a wall.
Paper clay that smells appalling
Normal, and it means the cellulose is breaking down. Use it sooner, make smaller batches, or dry it out and rehydrate as needed.
A weak fired pot
Too much of anything that burns out. Those voids stay voids. Paper clay and sawdust bodies are lighter and genuinely less strong.
Everybody's clay went gritty
Grogged scraps in the communal reclaim. Not fixable, and the reason modified clay stays in your own bucket.
The ones that come up at the tables
What does grog actually do?
It is fired clay, crushed, so it has already shrunk as far as it is going to. Mixed through a body it holds the structure open while the surrounding clay contracts, which lets water escape evenly and stops large or thick work cracking.
Can I just add sand instead?
Yes, and it does a similar job for less money. It is sharper on your hands, and because it is silica rather than fired clay it behaves a little differently at temperature. For most purposes it is a reasonable substitute.
Is paper clay worth the trouble?
If you hand build, yes. It lets you join bone dry to bone dry and repair cracks in dry work, which ordinary clay cannot do at all. It is also lighter, stronger while green, and it dries more evenly. It throws poorly and it rots if kept wet.
Will an additive change my glaze results?
It can. Anything you add changes the body's shrinkage, and glaze fit is a relationship between the two. A glaze that fitted before may craze on the modified version, so fire a test tile before a piece you care about.
Can I put grogged scraps in the reclaim bucket?
No. Grog blended into a smooth body makes everybody's clay gritty and it cannot be taken back out. Anything you have modified stays in your own bucket.
How much should I add?
Grog and sand, roughly 10 to 25 percent by dry weight depending on how open you want the body. Paper pulp, 10 to 20 percent by volume. Nylon fiber, a fraction of a percent. Bentonite, 2 percent or less. Weigh it rather than guessing.
Do burnout materials make the pot weaker?
Yes. Whatever burns away leaves a void behind, and those voids are permanent. That is the trade for lightness and for a pitted surface, and it is why these bodies are used for sculpture rather than for things that get handled hard.
Can I fire anything with additives here?
Mostly yes, with a conversation first. Heavy organics need a slower bisque and silicon carbide fumes through a whole load, so both change how a piece gets loaded. Telling somebody when you hand it in is the entire requirement.
Where to go next
Slam wedging
The technique for getting an additive evenly through a block, and how to tell when it is mixed.
Read it →Reclaiming your clay
Why modified clay stays in your own bucket, and what contamination does to a shared one.
Read it →What pottery glaze actually is
Why changing the body's shrinkage can craze a glaze that used to fit perfectly.
Read it →Masks and respirators
Weighing dry grog and powders is exactly the job the respirator is for.
Read it →Clay Hole University
Every lesson and guide in one place, from first day at the wheel through firing and glaze chemistry.
Start there →Two that matter, and a long tail
Strip this subject back and there are two additives that change what you can make. Grog, which lets you build big without the piece tearing itself apart as it dries. And paper fiber, which lets you join dry clay to dry clay and repair work that would otherwise be scrap.
Everything else on this page is either standard studio chemistry that arrives already in the bucket, or a decorative effect with a cost attached. Worth knowing they exist, worth trying if a piece needs one, and not worth adding to clay that was working fine.
Grog and sand open a body up so big work dries without cracking. Paper fiber lets you join dry to dry. Weigh what you add, wedge it properly, test a tile, and keep modified clay out of the communal reclaim.
If you want the full list, including glaze additives and what never goes in a shared kiln, switch to the Comprehensive Version at the top of the page.
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