Clay preparation

Slam Wedging: The Fastest Way to Fix a Bag of Clay

Cut it, stack it, slam it, repeat. It is louder and cruder than spiral wedging and it does a job spiral wedging cannot: it takes clay of uneven wetness and makes it one consistent material. This is how to do it, when it is the right tool, and why the slam has to be flat.

Cut the lump in half with a wire, stack one half on the other, and slam it down flat and hard. Twenty times. It blends uneven clay fast and it is the best thing there is for reclaim. Finish with a few spiral wedges to line the particles up.

Better than spiral for uneven clay Why the slam must be flat Three videos, none of them ours
Five steps When to use it Three things that go wrong
A potter cutting a block of clay with a wire and slamming the halves back together on a wedging table
Jump to a section
What it actually does When to reach for it How to do it The air question Your back, your wrists, your neighbors What goes wrong The whole idea, quickly The five steps Three things that go wrong Questions we get asked Keep going

What it does, when it beats spiral wedging, and how to slam without hurting yourself.

The whole idea

Cutting doubles the layers, every time

Cut a lump in half and stack it, and you have two layers. Cut that and stack it again and you have four. Then eight, sixteen, thirty two.

Twenty cuts gets you past a million layers. That is why a bag with a wet end and a stiff end comes out uniform after a couple of minutes of slamming, when kneading it by hand would take far longer and might never get there.

The slam is what welds the two halves back into one lump with no seam and no trapped air between them. A gentle press leaves a fault line; a proper flat slam does not.

Start to finish

The five steps

Slam Cut Wedging - Preparing The Clay - Pottery for Beginners Kara Leigh Ford
  1. Shape it into a rough block with a flat bottom. A round lump rolls when you slam it.
  2. Cut it in half with a wire, top to bottom. Look at the cut face; that is the only place you can see what the inside is doing.
  3. Stack one half on the other, cut faces out, and line the edges up.
  4. Slam it down flat and hard onto the table. Flat is the whole instruction. An angled slam folds air in rather than driving it out.
  5. Repeat fifteen to twenty times, then finish with a few spiral or ram's head wedges to align the particles before you throw.

Check a cut face every few rounds. When the streaks are gone, so is the unevenness.

If it went wrong

Three things that go wrong

The slam was not flat

Bringing the block down at an angle traps a pocket of air along the seam instead of squeezing it out sideways. This is the one mistake that turns slam wedging from an air remover into an air adder.

You stopped too early

Five cuts is thirty two layers, which is not enough to blend a genuinely uneven lump. Fifteen to twenty is the number, and it takes less time than it sounds.

You went straight to the wheel

Slamming blends the clay but leaves the particles pointing in every direction. A short spiral wedge afterward lines them up, and clay thrown without it fights you.

The mechanism

What slam wedging actually does

It is a different job from spiral wedging, and knowing which job you need is most of the skill.

Stack and Slam Wedging: Why and How to Stack and Slam Wedge Clay Expert Clay Four minutes on the why as well as the how, which is the part that decides when you should use it.
  1. It blends, above everything else. Each cut doubles the number of layers. Twenty cuts takes two lumps of different wetness and makes them a single uniform material, which is something no amount of kneading achieves as quickly.
  2. It drives air out sideways. A flat slam forces the two faces together from the middle outward, pushing air to the edges and out. That is why the slam has to be flat and firm rather than gentle.
  3. It does not align the particles. Clay platelets need to be pointed the same way for the clay to throw well, and slamming leaves them random. That is spiral wedging's job, and it is why the two techniques go together.
  4. It is faster than kneading for uneven clay, and slower for clay that is already consistent. If a bag is uniform and just needs waking up, spiral wedge it and skip this entirely.
  5. It is much easier on your wrists. Spiral wedging pushes through the heel of the hand repeatedly. Slamming uses your whole body and gravity, which is why it is the technique to reach for if your hands are complaining.
  6. It shows you the inside. Every cut is a cross section. Streaks, hard bits, air pockets and foreign objects all show on a cut face, and nothing else in pottery gives you that view.

The names, which are all the same thing

Slam wedging, cut wedging, stack and slam, wire wedging. Same technique, different studios. If somebody demonstrates one of these, you have learned all of them.

Choosing

When to reach for it

Slam wedging is the right answer for some clay and a waste of effort for the rest.

Reclaim, always

Reclaimed clay is never one consistency. Wet patches, stiff patches, and whatever was in the bucket. This is the technique that turns that into usable clay, and it is the main reason to learn it.

Blending two clays

Mixing a stiff bag with a soft one, or combining leftovers. Slamming is far quicker than trying to knead two different consistencies together.

Clay that has gone stiff on one side

A bag that sat with its top open, or a block that dried out at one end. Cut and slam redistributes the moisture that is already in there.

Large amounts

Twenty or thirty pounds is exhausting to knead and straightforward to slam. Gravity does the work that your shoulders would otherwise do.

When your wrists hurt

Spiral wedging is repetitive load through the heel of the hand. If that is an issue for you, slamming is the alternative that still prepares the clay properly.

Not for fresh, even clay

A new bag straight from the box is already uniform and already de-aired. It needs a short spiral wedge, not twenty slams. Doing this to fresh clay is just noise.

Doing it

How to do it properly

Stack and slam wire wedging The Pottery Wheel A clean, slow demonstration with a good view of the cut face between rounds.
  1. Work on a surface that can take it. A plaster or canvas-covered wedging table. Not a slab roller bed, not a wheel, and not a folding table that will give way.
  2. Shape the lump into a rough rectangle with one flat face. Round lumps skid off the table when you slam them.
  3. Cut with the wire in one clean pull. A sawing motion leaves a rough face that traps air when the halves go back together.
  4. Look at the cut. Streaks mean it is not blended. Holes mean air. Anything dark and gritty means something got into the clay and should come out now rather than in the kiln.
  5. Stack the halves square, cut faces outward, edges lined up. A sloppy stack produces a sloppy slam.
  6. Slam flat, from a decent height, with the whole face landing at once. You want a proper crack of sound, not a thud.
  7. Reshape between rounds so the block stays square. A block that spreads sideways stops cutting cleanly.
  8. Finish with spiral or ram's head wedging, five or six turns, to align the particles before throwing.

The number that matters

Fifteen to twenty cuts. Fewer and the blending is incomplete; more and you are just making noise. Judge it by the cut face rather than by counting, and stop when the streaks disappear.

The argument people have

The air question

Slam wedging gets described as both the best way to remove air and a good way to add it. Both are true, and which one you get depends entirely on the slam.

  1. A flat slam removes air. The two faces meet in the middle first and the contact spreads outward, pushing air ahead of it and out at the edges.
  2. An angled slam traps it. If one edge lands first, the faces close from one side to the other and seal air in along the seam. That is the whole difference, and it is why every demonstration makes a point of the slam being flat.
  3. Air bubbles are less catastrophic than people say. The old warning that a bubble makes a pot explode in the kiln is mostly folklore. Bubbles cause bumps and wobble on the wheel, which is a real nuisance, and pots explode from being wet, not from air.
  4. Where they do matter is throwing. A bubble under your fingers as a wall thins is a weak spot and a wobble, and it is genuinely annoying on a tall form.
  5. You can see them on the cut face. Every cut is a free inspection. If holes are still showing after fifteen rounds, your slam is not flat.
  6. A pug mill with a vacuum does this properly. If you have access to a de-airing pug mill, it removes air more completely than any hand technique. Slam wedging is the manual version of the same idea.
Practical

Your back, your wrists and your neighbors

Slam wedging is easier on your hands than spiral wedging and harder on everything else if you do it badly.

  1. Table height matters more than technique. A wedging table around hip height lets you use your body weight. Too low and you bend; too high and you cannot get any force into it.
  2. Drop, do not throw. Lift the block and bring it down with weight rather than muscling it. Your arms are guiding; gravity is doing the work.
  3. Keep your back straight and hinge at the hips. The injury from this technique is always a back, never a hand.
  4. Twenty or thirty pounds is a two-handed lift every single time. Splitting a large amount into two manageable blocks is faster than struggling with one.
  5. It is loud. Genuinely loud, and in a shared room it carries. Worth a glance around before you start, especially if somebody nearby is centering something delicate.
  6. Watch the wire. A cutting wire under tension near your hands is the small hazard nobody mentions. Pull it through the clay, not toward your fingers.
Read this after a bad lump

What goes wrong

Streaks still showing after twenty rounds

The two clays are too different in wetness to blend, or you are cutting the same way every time. Rotate the block ninety degrees between cuts so you are not just re-stacking the same layers.

More air than you started with

Angled slams. Watch where the block lands. If one edge hits first, you are folding air in with every round.

The block will not stay square

Too soft, or you are not reshaping between rounds. Very wet clay spreads on impact and stops cutting cleanly. Let it stiffen on a plaster bat first.

The wire keeps snagging

Something hard is in the clay. Stop and find it. Usually a dried chunk, occasionally a bit of plaster, which matters because plaster in clay blows a hole in the pot at temperature.

It throws badly afterward

You skipped the spiral wedge at the end. Slamming blends but leaves the particles unaligned, and that is what makes clay feel unresponsive on the wheel.

Your back hurts

Table height or bending. Hinge at the hips, keep your spine straight, and if the table is too low, that is worth raising rather than working around.

Questions we get asked

The ones that come up at the tables

Is slam wedging better than spiral wedging?

Different, not better. Slamming blends clay of uneven consistency far faster. Spiral wedging aligns the clay particles, which is what makes clay throw well. Most people use slam wedging to fix a lump and then spiral wedge it to finish.

How many times do I need to cut and slam?

Fifteen to twenty for reclaim or badly mixed clay. Judge it by the cut face rather than counting: when the streaks have gone, so has the unevenness. Each cut doubles the layers, so twenty cuts is over a million.

Does slamming remove air or add it?

Both, depending on how you slam. A flat landing squeezes air out to the edges. An angled landing seals it in along the seam. This is the whole reason everyone insists the slam be flat.

Will an air bubble make my pot explode?

Almost certainly not. Pots explode because they went into the kiln damp and the water turned to steam. Air bubbles cause wobble and weak spots while throwing, which is reason enough to get rid of them, but the explosion story is mostly folklore.

Can I slam wedge on any table?

No. Use a proper wedging table, plaster or canvas topped, at roughly hip height. Slamming twenty pounds repeatedly onto a folding table or a slab roller bed damages the equipment and eventually the table.

Do I need to do this to fresh clay from a bag?

No. Commercial clay arrives mixed and de-aired. A short spiral wedge to wake it up is all it needs. Save slam wedging for reclaim, for blending, and for anything that has dried unevenly.

Why does my wire keep catching on something?

There is something hard in the clay. Stop and get it out. Usually it is a dried lump, sometimes a tool fragment, and occasionally plaster, which is the one that matters, because a piece of plaster in a pot expands at temperature and pops a crater out of the wall.

Is it bad for the studio to be that loud?

It is not a rule, it is a courtesy. Slamming carries across a room, and somebody centering a tall piece will feel it. A glance around before you start covers it.

One last thing

Two minutes that save an hour

Most of the clay that fights people on the wheel was never properly prepared. It had a wet end and a stiff end, or it was reclaim that somebody kneaded a few times and called done. The pot goes off center, the wall thins unevenly, and the conclusion is that today was a bad day at the wheel.

Slam wedging is the two minutes that prevents that. It is loud, it is not elegant, and it is the single most effective thing you can do to a difficult lump of clay. Cut it, look at the face, slam it flat, and keep going until the streaks are gone.

Cut, stack, slam flat, fifteen to twenty times, then spiral wedge to finish. Best thing there is for reclaim and unnecessary on a fresh bag. Flat slams remove air, angled slams add it.

If you want the full picture, including the air argument and the six ways it goes wrong, switch to the Comprehensive Version at the top of the page.