Swoop Bowl

This form is a variant of the bowl that was made from an annealed aluminium blank in the shape of an equilateral triangle with rounded corners. When hammer-formed into a bowl, the three corners lift up to give a dynamic and swooping feel.

What you need

• Permanent marker
• Scrap paper
• Annealed aluminium sheet
• Metal shears
• Metal guillotine (optional)
• Wooden stump
• Clamps
• Hammer-forming sandbag
• Assortment of files
• Delrin mallet in teardrop shape
• Delrin mallet with flat surface
• Planishing hammer with flat surface
• Large domed planishing stake
• English wheel (optional)
• Shallow tray with some water inside
• Assortment of sandpaper (400, 800, and 1200 were used)
• Brasso
• Rags

1. Preparing the blank

Taking your sheet of annealed aluminium, mark out an equilateral triangle with rounded edges using a permanent marker. Cut out the triangle-shaped 'blank' with a metal guillotine and shears.

Prepare the workpiece for filing by clamping it between two sheets of softer material, and begin filing along the direction of the metal's edge. This reduces the chances of the file catching on the edge of the aluminium, which would cause it to 'skip' and  be harmful to your workpiece or yourself.

Next, mark out a spiral that begins in the centre of the blank and expands to fit the edge of the blank.

This is a rough guide for the initial shaping, as we will be stretching the centre into a bowl shape.

2. Preparing the workspace

Clamp the wooden stump to the bench, ensuring the clamp does not get in the way of the workpiece. If the workpiece is struck while resting on the clamp, large dents and scratches will be made.

Flatten out the sandbag, then beat the centre with a hammer to form a bowl shape. The sandbag is much less rigid than the wooden stump, but allows for more freedom of movement.

  

3. Initial shaping - stump shrinking

Position the centre of the blank over the bowl of the wooden stump and strike the central ring of the spiral with the pointy end of the teardrop mallet.

Do not use excessive force, only enough to dent the metal moderately. Dents which are made with too much force (pictured below) have imprinted marks from the mallet, and are prone to splitting due to the excessive stretching of material.

The type of mark left by striking with excessive force.

As you continue to shape the blank by striking along the spiral, the outer edges of the metal will form large creases, which are called 'tucks'. These are vital to the creation of the bowl shape, but must be treated with care. Details in the next step.

4. Refining and fixing tucks

Tucks should be shaped into tight creases to allow them to develop as the material is shaped further. This is done by striking the sides of the tuck with moderate force perpendicular to the metal (pictured).

Strike the sides of the tuck to tighten it.

Successfully tightened tucks.

As you continue to shape the workpiece along the spiral, more and more tucks will appear as the metal on the edges tries to push into itself. Tighten these tucks as they appear. 

More tucks have appeared as the shaping continued.

If you continue to shape the metal without tightening the tucks, they may grow further until the metal folds over. This may also happen to normal tucks if you strike with enough force or in a certain place. 

To fix these folded-over tucks, use the pointed end of the mallet to strike them back into a flatter shape. Turning the workpiece over and using a wooden stake can also help greatly (pictured).

A tuck starting to fold over.

Using the edge of the shrinking stump as a stake.

The repaired tuck.

If this does not work, stretching the material on the shrinker/stretcher may help. When using the shrinker/stretcher, place sandpaper between the workpiece and the machine jaws to reduce surface marring (pictured).

Some 400-grit sandpaper.

Sandwich the workpiece between the pieces of sandpaper before using the shrinker/stretcher.

Before refining tucks. 

After refining tucks - note the undesirable 'S' shaped fold in the centre.

After stretching the material. The 'S' shaped fold is greatly diminished.

5. Further shaping

Continue striking along the spiral with the pointy end of the teardrop mallet, refining and flattening the tucks as you pass them. Return to the centre to restart the spiral once you have reached the edge. The more passes of the spiral you do, the deeper the bowl will become.

The tucks have been flattened to a moderate level. It is not necessary to totally flatten them at this stage.

As the workpiece becomes more bowl-shaped, it is necessary to change how you position it on the sandbag or the shrinking stump. Positioning the striking area to face away from you in the bowl is viable but counterintuitive, as the mallet 'digs' towards the centre of the workpiece. It is more effective and intuitive to position the striking area so it faces towards you (pictured).

Here, the striking area is positioned away from you.

Here, the striking area faces you directly.

Be aware of material fatigue, also: once you feel the aluminium resist shaping at a much greater level than before, return it to the oven for annealing. This involves leaving it in the oven at 375 degrees Celsius for ten minutes. After this, the material should be back to its original level of malleability. If you feel the material work hardening but fail to anneal it, there is a chance the material will split.

It is important to get the shape right at this stage. In the picture below, the corners of the workpiece were shallower than the flatter edges. I used parallel lines to mark out the 'zones' which needed further stretching, and focussed striking those areas.

Marking the workpiece to show which areas need to be deeper.

6. Detailed shaping

When the desired depth and overall shape is found, use the round end of the teardrop mallet to flatten out the bumpy surface and tucks. Use a low-to-moderate force for this stage, as the bumps tend to level quite easily. Using excessive force will stretch the material too far, which will be hard to bring back to its original level.

Ready for English wheel.

When the majority of the bowl has been roughly evened out with the teardrop mallet, you can move to the English wheel to smooth the surface further.

Ensure that the anvil on the English wheel is the right size, and does not deform your bowl when you try to roll it through. Also, make sure the anvil is not too deep, as that may cause your bowl to stretch further than desired.

This anvil is too deep.

This is a good sized anvil.

Rolling the edge to flatten tucks.

Rolling the centre to smooth the surface.

The tucks were not completely smoothed out.

7. Planishing

Use a flat-surface delrin mallet to flatten the tucks against the domed stake, as much as you can, before moving onto the planishing hammer. This will reduce the amount of marring on the underside of the bowl, and reduce the stretching of material so that the bowl does not deform much at this late stage.

The steel planishing hammer, due to its physical properties, can leave huge dents in the soft aluminium if you are not careful. As such, I advise you to use minimal force when planishing. Using little force aids you in keeping the striking surface flat.

Try to keep a steady rhythm with the planishing hammer as you move the bowl around on the domed stake with your free hand. Don't tense up or you will get tired quickly. Instead, the hammer should bounce back with each hit if you are using the right amount of force.

Listen for the distinct ringing sound of metal on metal. If there is an empty, tinny sound instead, it is likely that the bowl is not in contact with the stake when it is struck, meaning it is deforming in an undesirable way. This can be fixed by continuing to planish around the deformed area.

Use the delrin mallet to do as much shaping as you can before moving to the planishing hammer.

Be careful when using the planishing hammer.

8. Cutting to size

Due to difficult-to-control nature of hammer-forming, you may need to trim one or two of the sides to fit the others. Trace the smallest corner onto a piece of paper, then cut it out and use it as a template for the other two corners. Following this, mark the desired corner size with a marker, then cut them out with metal shears. File the edges down again, using a file with a curved edge for the inner edges of the bowl.

The smallest side.

9. Sanding

Wet-sand the bowl by dipping it in a shallow tray of water and sanding in circular motion with 400-grit. This will remove many of the smaller scratches, yet retain the planished look. When you are satisfied, rinse the bowl, and repeat with 800-grit, then 1200-grit, and higher if you wish .

Covering the bowl in water.

Notice the aluminium slurry left behind from sanding.

Rinsing the workpiece.

10. Polishing

With a soft rag, apply a small amount of Brasso to the aluminium in a circular motion. This should turn the materal dark grey. With another rag, polish out the Brasso with quick, circular motions. This is quite tough to do, especially on an unwieldy object such as this. Persistence is key to this step.

Complete

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