Technical Thursdays - Repairing a fault in a bowl archive

Thursday 15 May 2014

Steve Bisco shows you how to cover up that crack in your bowl

Gallery

It is sometimes said that problems are just opportunities in disguise, and most woodturners who have tried to dry green wood will have been presented with the 'opportunity' to deal with large shrinkage cracks in their part-turned bowl blanks.

With a small split, you may get away with filling the crack with a paste made of fine shavings from the same wood, mixed with PVA adhesive. With a large split, however, filling it may just produce an unsightly scar in an otherwise pristine bowl. My father used to say 'If you can't hide it, make a feature of it', and that is how I like to approach an 'opportunity' in a bowl blank. I just make the crack bigger and line it with gold! It gives the impression that a solid gold bowl has been concealed within a wooden skin, and the trick has been revealed when it has split open.

A lot of people think gilding would be beyond their skills, but it is really quite a simple process, as I hope to show in this article. Imitation gold leaf is cheap and easy to obtain from art stores and is also quite easy to handle. Its only drawback is that it is basically brass foil and needs sealing with shellac, artist's varnish, or another clear 'skin' to prevent it from tarnishing. Real gold leaf of 18-24 carats, gives a truly golden finish that will stay bright forever without any sealer, but it costs a lot more, and is harder to handle. I will show you how to use both mediums, so you can progress from the economy option to the luxury finish if and when you want to.

Working with cracked pots

STEP 1

I rough-turned a green log of cherry wood (Prunus spp.) into some bowl blanks, sealed them with Danish oil and put them away to dry for a year. While others of the batch were mostly OK, this one is split from top to bottom on one side. Apart from the split, the wood is attractive and it would be a pity to waste it, so it is time to mount a rescue operation

STEP 2

To work on a split blank, you need to be able to grip the base in compression mode. Using your chuck in expansion mode would open the split further and the grip would be unsafe. If you can't fit the whole foot of the bowl in the chuck, then cut a spigot into the base so you can grip it safely in compression mode. Here I am using the tailstock to steady the foot, while a round piece of wood in the chuck is providing a friction drive to the part-turned bowl blank. Lathe speed is about 200-300rpm

STEP 3

Always use whole-face protection. Keep the lathe speed low when turning a split blank to minimise the risk of the blank coming apart

STEP 4

With the outside of the bowl turned, it is clear that hiding this crack is not an option! We now have a 'golden opportunity' to turn it into a feature

STEP 5

With the bowl held securely in compression mode by the spigot, in the base, we can finish turning the rim and interior. A good wall thickness will give a larger area to splash with gold - and will be stronger - so if you love to show off your thin-walling skills, restrain yourself when working with a cracked bowl

STEP 6

Sand the bowl while it is still held in the chuck. Hold the abrasive so it flows over the crack, with your fingers pointing 'downstream' to the direction of rotation. Apply your chosen finish. If you apply wax with the lathe rotating, always hold the cloth between fingers and thumb so it can snatch out without harming your fingers if it catches

STEP 7

When the chuck work is finished, remount the bowl using the tailstock and friction drive - as in step 2 - and turn away the spigot in the foot

Preparing the crack for gilding

STEP 1

Hold the bowl gently in a vice, protected with padding, and use a coping saw to cut out a wider crack. Splay the sides of the crack outwards to create a greater surface that is easier to get at and give it an interesting convoluted shape

STEP 2

Use a rotating multi-tool and abrasives to enhance the convoluted surface, and smooth it

STEP 3

You can now seal the bare wood with a sanding sealer and, when dry, use fine abrasives to get the surface really smooth

The economy option: gilding with imitation gold leaf

STEP 1

The cheapest and easiest way to gild is by using imitation gold leaf, which is basically brass. It is sold at most art stores, in books of 25 sheets, for about £10. Each sheet measures 140mm square, so a book can cover a lot of wood, even allowing for wastage. You will also need some gilding size, a brush to put it on with and a soft brush to press down the leaf

STEP 2

Apply some gilding size thinly to one side of the crack and leave it for about 10 minutes until it is dry, but so it is slightly tacky to the touch. The leaf will stick exactly where you put the size, so take care not to get it on the surface of the bowl. If you do, then wipe it off with a damp cloth

STEP 3

Take a sheet of gold leaf, still in its cover papers and cut it into pieces about 50mm ? 25mm. Fold a small piece of paper and use it to pick up a piece of leaf, then slowly and carefully place it on the sized area. Press it down gently with a soft brush and brush away any loose pieces of leaf. This is a bit tricky at first, but you will soon get the hang of it. Continue over the whole area, and then go over any gaps with more leaf. Repeat the whole process on the other side of the crack

STEP 4

Imitation gold leaf needs a sealer to stop it tarnishing. I generally use clear artists' varnish - as here - or French polish if I want to 'antique' the gold

STEP 5

The ugly scar of the shrinkage crack has now been replaced with an intriguing flash of gold

The luxury option: using real gold leaf

STEP 1

Some jobs are worth going the extra mile and there is nothing quite like the glow of pure gold. Real gold leaf - I use 23? carat gold - also comes in books of 25 sheets, but each sheet measures only 80mm square, which is a third of the area of an imitation leaf. The price of gold fluctuates frequently, but at the present time expect to pay around £36 for a book, which you will need to get from a specialist supplier - I buy mine online from Alec Tiranti. You will also need a 'gilder's tip' brush to handle it and, as with the imitation leaf, some gilding size, a brush to put it on with and a soft brush to press down the leaf

STEP 2

Apply a thin coat of size and leave it to become touch dry. This should be done in exactly the same way as for imitation leaf

STEP 3

Real gold leaf is much thinner than imitation gold and the first time you try gilding with it, you will find it frustrating, but with a little practice you will soon get the hang of it. Carefully fold back the cover paper on a sheet of gold leaf - it is very delicate - and gently score across the leaf with a knife at the edge of your fold. Cut a piece about 25mm x 50mm, pick it up slowly and carefully with the 'gilder's tip' - rub it in your hair first to build up static - and lower it carefully into position. Press it down gently with the soft brush

STEP 4

Brush away any loose leaf from the edges and go over bare patches again with small pieces of leaf. This bowl now has a heart of pure gold that will never tarnish. It doesn't need sealing and will retain its brightness for many years


Woodworkers Institute

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Steve Bisco , Technical Thursdays


Health & Safety

With any timber that has cracks, fissures, voids or similar, the structural integrity of the piece is impaired. Only work with such timber if you truly have to and if you do, wear appropriate PPE, keep the lathe speed low, take gentle cuts and try to reinforce the work by using adhesive tape, wrapping the piece in clingfilm/food wrap, or similar techniques to minimise the risk of the piece coming apart when working on it.