A Cracking Finish:
Adrian Jacobs shows how to use crackle glaze to maximise the visual impact of bowls.
I have written in Woodturning before about the use of crackle glaze and how, when used sensitively, it can really enhance an otherwise unexciting bit of wood. Over the past few years I have been experimenting with the finish and this article sets out some of the tips and techniques you can use to ‘jazz up’ your turning. Recently, I was given a board of softwood, a kind of pine (Pinus spp.), that turns and finishes really well but
it had lots of faults and shakes in the wood. I planned to make some large bowls out of it, but in the end I had to settle for five blanks of about 280mm diameter. At the same time, I was about to dispose of three very tatty rubberwood picnic tables that had supported plants in my conservatory. They were stained beyond redemption and had been condemned by the domestic chief executive. I never throw wood away unless I am absolutely convinced that I can’t make use of it and, after a bit of thought, I decided to use these with the pine blanks to make a large bowl and the rubberwood rim that could be covered up by crackle glaze. This article shows you the tips and techniques you can use to make a similarly large bowl, and what you need to know about using crackle glaze.
What is crackle glaze?
There are a number of crackle glaze products on the market, but the one product I have had most success with is the Plasti-kote range. The products work by spraying on a base coat of an enamel-based paint, followed by a top coat of ‘crackle glaze’ that splits and cracks as it dries, revealing a network of cracks that exposes the underlying base coat. The base coat and top coat are chemically incompatible and that encourages the cracking that can be used to give quite dramatic effects. The Plasti-kote range is quite limited in colour combinations and, although it used to be widely available in stores such as B&Q,
I have found more recently it is less easy to get hold of and I have had to top up my supplies through internet purchases.
Plasti-kote now supply top coats in black, heritage gold and cream, and base coats in gold and brown, but I have discovered you can use almost any enamel-based spray paint as a base coat. This give you access to a much wider range of crackle effects. Plasti-kote also do a wide range of enamel spray paints and these work very well, as do enamel-based car spray paints from Halfords.
The finished rubberwood and elm (Ulmus procera) bowl I mentioned in the opening paragraph was made using a base coat combination of red and blue enamel, and a heritage gold top coat. The rims of this bowl are a simple gold spray paint.
For the more adventurous among you, it is possible to use acrylic-based paints as the base coat, as long as you cover these with a couple of coats of clear enamel before spraying with the top crackle glaze. The clear enamel spray is available from most DIY, craft and car repair outlets.
Health and safety
• MDF has a reputation for being particularly unsafe in dust form, but a search of the internet suggests that it is no more unsafe than any other wood dust. As always you should take precautions to protect yourself from dust using masks and extraction systems.
• The use of solvent-based lacquers poses two risks: they are very flammable so do not use near any naked flames; the fumes can be toxic so make sure the workshop is well ventilated.
Bowl design is fairly straightforward and if you follow the ‘golden ratio’ of decoration to plain wood (roughly 1:1.6) you will probably not go wrong. Despite this, do not be afraid to experiment, as the late great Douglas Bader said, ‘rules are for the guidance of wise men and the obedience of fools’. The most important thing is that there should be sufficient width – typically a rim section on platters, but it can be the outside of bowls, hollow forms and such like – to allow crackle-glazed section to have visual impact.
The bowl with the red and blue base coat and a Heritage gold top coat (Fig.1) has an overall diameter of 325mm crackle pattern width of 70mm.
The larger of the two black and gold bowls (Fig.2) has an overall diameter of 360mm and a crackle width of 65mm and the smallest (Fig.3) has an overall diameter of 270mm and a crackle width of 45mm.
I think that they all work although may not quite follow the golden ratio.
You do not need to confine yourself to using crackle glaze on wide rimmed bowls. It also works well as a side band on a simple bowl. Here (Fig.4) you can see the small bowl that has a crackle band on the inside and the outside; I did this to cover a nasty knot in an otherwise nice piece of cherry (Prunus spp.) that was destined for the firewood pile. Here (Fig.5) shows a small bowl made out of the same cherry tree with a simple external crackle band.
Tips and tricks
• Before you trim the masking tape, run your fingernail around the point where you are going to trim to ensure that the masking tape is firmly adhered. Do this again after you have trimmed the tape to ensure the paint cannot creep underneath.
• If, after removing the masking tape, you find there is overspill onto the masked area, then you can touch this up with a fine paint brush using spray paint that has been sprayed into a small container.
• If you have accidentally applied too much spray and there is a danger of runs, use your variable speed (if you have it) to run the lathe at a very low speed while the paint dries. This prevents gravity from dragging the paint at a single point.
• If the masking tape leaves any residue, use white spirit as a solvent to remove it. Cellulose thinners will damage the finish.
When creating a wide-rimmed bowl, you can either have a supported rim (Fig.6) or a ‘floating rim’ (Fig.7). The supported rim will be much heavier and requires a large blank because it all needs to be made from one blank. The ‘floating’ rim will be much lighter in weight and you can combine all sorts of wood and man-made materials to create it.
Creating floating rim bowls on small blanks is fun, gives you the opportunity to experiment and can achieve dramatic results from small wooden blanks. I have done this in three ways using; a disk of wood reclaimed from an old table; a disk of wood cut from a piece of 11mm thick MDF; and a segmented ring. Whichever way you do it, I recommend following more or less the same process.
Turning your wide-rimmed bowl
Rough turn a bowl to more or less the dimensions that you want the finished bowl, but leave it 2–3mm thicker than you would normally do. Make sure the rim is dead flat (Fig.8) to ensure that there is no gap between the rim and the bowl when you glue up. Then, cut a step about 5mm wide and 3mm deep (Fig.9) in from the outer rim (Dia.1). This step will form the basis of the joint with the rim.
Do’s and don’ts
• Keep the workshop as dust free as you can before using the paint sprays. It pays to vacuum the work shop and use a dust extractor, then leave the shop to settle for as long as you can. I try to leave it at least an hour before I spray.
• Try to use the most stable timber that you can find, well seasoned quarter sawn blanks are ideal but even they can move after turning. I find that I get the best results by rough turning blanks and leaving them aside for a few months to let them move. When they are finally returned they are almost totally stable and if you are fixing artificial rims to them there is almost no risk of unwanted gaps appearing after a few months.
• Use a wide enough band of crackle glaze to give it some impact, small bands simply do not look good.
• Let the base primer coats dry well between coats to avoid runs, and keep the temperature as high as you can in the workshop – it will speed drying/curing times.
• Allow the base coat crackle glaze to dry so that it is touch dry (usually about 10-15 minutes) before applying the top coat. Don’t let the base coat dry out too much so get the top coat on within about an hour.
• When spraying the final coat of clear lacquer, follow the instructions on the can as far as timing between coats goes. Different products vary, I avoid water based products because of the longer drying time and they tend to run more easily!
• After the final coat of lacquer, leave the bowl to harden for a couple of days if you can. This will help to avoid damage when you reverse it to clean up the base; soft lacquer damages easily.
• Be prepared to experiment with colour combinations and thickness of the top coat.
• Keep the piece on the lathe as long as you can. If you don’t like the finish it is easy to scrape it off with a bedan and start again.
• Overwhelm a beautifully figured wood with complex finish, the final product will be confusing to the eye. The more the figure in the wood, the simpler any enhancements should be.
• Rush your paint spraying and drying times. Patience will reward you with a better finish.
Joining the rim and body blank
Mount the rim blank on the lathe. Any type of mount is fine but I use a screw chuck because it is quick and easy to use. Turn the rim flat and mark out a circle of the exact diameter as the stepped part of the bowl.
If you are using a segmented ring, glue this to a piece of ply or MDF (Fig.10) and mount the MDF on the lathe. After truing up the disk, turn away about 2mm of the surface leaving an outer ridge that you will use to form a bead on the rim of the bowl, then create a dead flat surface around the glue joint. I use a bedan to do this and I get a very good finish if I take very light cuts. Here (Fig.11) you can see an MDF blank with the outer ridge and a second inner ridge which is where the bowl will finally sit. Turn away the centre of the blank to the exact diameter of the step on the bowl blank. A tight fit is important so creep up the final diameter until you have a push fit that is almost as good as if you were using a jam chuck. Glue the bowl to the rim and bring up the tail stock to hold everything in place while the glue dries.
Refining the outside of the bowl or platter
Finish the outer surface of the bowl to its final dimensions. I like to leave a small bead around the glue joint and Fig.12 shows an MDF/elm combination with just such a bead. Don’t forget to leave a spigot to fit your chuck jaws for turning the inside of the bowl.
Sand the bowl through the grits to get the best possible finish and give everything a coat of full strength cellulose sanding sealer. Wipe off the excess and buff with ‘0000’ wire wool and then with a soft cloth. If you are using MDF for the rim it sometimes helps to give it two coats of sanding sealer because it is very absorbent. You are now ready to mask the underside of the bowl.
Preparing and spraying
Cover the lathe to protect it from over spray and mask up the underside of the bowl so that you can spray the underside of the rim with the finish that you will use for the bead on the rim. Here (Fig.13) is the tape in place. Use your fingernail to make sure the tape is firmly adhered to the surface before you trim the tape. Set the lathe speed to low and use the point of a skew chisel to cut the masking tape where you want the delineation of the finish. Peel away the excess tape (Fig.14) and you will be ready to spray. Apply the spray in several thin coats. In Fig.15 I have used gold spray and I am buffing with ‘0000’ wire wool between coats in order to get a good finish. When you are happy with the quality of the finish, peel away the masking tape and you are ready to re-chuck and turn the inside of the bowl (Fig.16).
Re-chuck the bowl and turn to its finished dimensions and leave a bead on the inner and outer side of the rim. Finish with sanding sealer and buff with wire wool. In Fig.17 and Fig.18, they show the glue joint between the rim and the bowl. You are now ready for two-stage masking procedure.
Mask up the inside of the bowl so that you can spray the inner and outer rim. Cut the masking tape with the tip of your skew chisel exactly on the glue joint (Fig.19). Remover the tape and spray with your chosen rim colour. This photograph (Fig.20) shows the finish being sprayed on. On this occasion, I am planning to use a gold base coat and so I have sprayed the whole rim. Leave the paint to dry and you can start the next stage of masking.
Mask up the beads and cut the masking tape with your skew on the outer edged of the inner bead and the inner edge of the outer bead, strip away the masking tape on the flat surface of the rim leaving it clear for the base coat of paint.
Spray on your base coat in layers. If you are using Plasti-kote crackle glaze base coat or any other enamel-based paint, then I recommend that you use three light coats.
If you are using a non-enamel based paint, then I suggest two coats followed by two coats of clear enamel.
After the paint is thoroughly dry, apply your preferred polish to the natural wooden surface of the bowl, re-chuck and turn away the tenon on the base and apply a final coat of polish to the outside of the bowl. I use a vacuum chuck to do this but if you are using a large face plate or friction drive, take care not to damage the crackle glaze.
Sometimes you will need to fill imperfections in either the bowl or the rim. I have used all sorts of concoctions to do this, most of which are based on an old French polisher’s trick of mixing Danish oil and poster paint to use as a grain filler. I am not keen on using Danish oil under enamel-based paint and find that cellulose sanding sealer is a good alternative and dries much more quickly. Other combinations that work are: acrylic paints and powder paint; polyester resin and powder paint (although this does set very hard); Melted bee’s wax and powder paint.
There are very many commercial products that you can use but I find that Ronseal multipurpose wood filler works very well, dries quickly, is available in dark, medium and light colours and a range of sizes from small tubes to big tubs.
I hope that this article has stimulated your interest in experimenting both with the technique of creating wide rim bowls from leftover wood and the use of crackle glaze. There are many factors that you can alter to create impact to you work. These include:
• Bowl shape with or without undercut rims for the inner bowl.
• Rim width and bead design.
• Colours and types of base coats.
I particularly like the effect that you can get from metallic base coats made by Valspar. They do need the addition of a ‘sandwich’ coat of clear enamel between them and the crackle glaze but I think they work really well and will be using more of them in a wider range of colours. I also now have a smaller fire wood pile because many of the offcuts that used to throw away will now be pressed into service to make segmented rings for the rims of these bowls.
So go ahead and try it out; as ever the only limit is the rather small range of Plasti-kote crackle top coats and your imagination. If anyone out there knows how to create their own crackle top coat please get in touch, I am dying to know how to do it.
Alternative bowl designs