One Big Dovetail archive
Thursday 6 October 2011
Michael Huntley introduces and mixes shellacError loading Partial View script (file: ~/Views/MacroPartials/cwsGalleryImages.cshtml)
All professional polishers use shellac. This is why I am writing about shellac before writing about some of the other surface finishes. Shellac is the most versatile of the surface films available to furniture makers. It does have disadvantages, specifically that shellac is neither water nor alcohol proof, but the ease with which the surface can be repaired provides an antidote to these problems.
Shellac is obtained from the secretions of a scale insect called Laccifer lacca. This insect lives in certain trees found in south-east Asia. Note that these are secretions not excretions - some people are worried about the origins of shellac. The secretions of resin accumulate on twigs and these are harvested. At this stage, stick-lac, there are various impurities in the harvested material. The sticks are crushed and sieved to remove the larger contaminants. The crushed material is washed and sieved again. This results in seed-lac.
The refinement process is continued by heating the seed-lac in bags and squeezing the resin out through the cloth. It is then stretched into a thin sheet. This sheet is broken up to produce shellac flakes.
This is the modern way of producing shellac. It is worth noting that shellac coatings have been found on turned objects of considerable age. The process then was to use the friction of turning to soften the shellac and cause it to adhere to the turning.
Making your own shellacThe shellac resin can be dissolved in alcohol. This is a very useful property. You can buy shellac ready mixed with alcohol or you can mix your own. Be aware that shellac has a shelf life, in that while old shellac does not go off in the sense that you cannot use it, its working properties are just not so good.
I use old shellac as a sanding sealer or as a sealer on jigs and other projects that do not need to have a perfect finish. Mixed shellac from a good polish house should be used within a year. Once that year is up, buy a new bottle and use the old stuff around the workshop.
If you mix your own polish then you can mix up as much as you need, thereby always having fresh polish and avoiding waste. Mixing the polish is simple. Put some alcohol in a jar, add some shellac flakes, put the lid on and forget it. The flakes will take between 24 and 48 hours to dissolve. It helps if you give it an occasional shake.
QuantitiesThere are various recipes for shellac to alcohol such as a 2lb cut. This is an old, but perfectly acceptable, way of describing the concentration of shellac flakes to liquid alcohol. It means 2lb of shellac to a gallon of alcohol. Most of us do not have a gallon of alcohol, so an easier ratio is 100 grams to 500 millilitres. There are other concentrations but start with this one.
Always label your containers with the type of material, the concentration and the date made. In due course you will end up with some bottles containing stronger concentrations and coloured polishes.
ContainersPolish used to be kept in wine bottles with a V-groove cut in the cork. Nowadays plastic bottles are easily obtainable. Some people buy new bottles with a little plastic straw applicator in the lid, but I use old Fairy Liquid bottles, which are free. The bottle must be absolutely clean, so I wash them 10 times and then rinse them out with some special old meths kept for the purpose.
Pay particular attention to the stopper because the tiniest smear of soap will ruin your polish. The meths will have been used for cleaning brushes for a few weeks, then for cleaning polish bottles before being thrown away.
Try and recycle your solvents as much as possible, it saves money and saves the planet.
Types of shellacThere is a lot of confusion about what type of shellac to use. The simple answer for all those who have not used shellac before is Special Pale. This easy-to-apply shellac does not have an over-strong colour and is what students at West Dean College, near Chichester use.
My polish supplier is Mylands whose catalogue quotes the following shellacs in liquid form,
French Polish - rich medium brown in colour and good for walnut, oak and mahogany
Button Polish - has a slightly warmer appearance than French Polish
Superfine White Polish - used for pale timbers such as oak, ash, beech and sycamore, not suitable for high-wear situations like table- and desktops
Special Pale Polish - a classic pale polish suitable for most projects
Garnet Polish - cold brown colour and the darkest of all polishes
Hardspar Polish - clear to transparent polish, slightly more durable than Special Pale, but harder to use
Black Polish - for tinting other polishes
Red polish - for tinting other polishes
Other typesThere are two other classes of shellac polish, modified shellacs and sanding sealers. Modified shellacs contain additives to improve the heat resistance or durability. Once again, these are a little harder for the beginner to use. Sanding sealers are intended to seal the surface and provide a high build that can be sanded back to provide a base for the finishing polishes. Duraxcoat and Wuncoat are sanding sealers.
Blonde shellac, Garnet, Lemon and Waxy White are all types of shellac flakes that can be used to make up your own polish. I use de-waxed blonde flakes.
Making your own polishI said earlier that shellac can be dissolved in alcohol. Polish houses sell special alcohol called finishing spirit that can be used to thin down shellac polish or indeed to use as a base for making your own polish. You can also use meths, but that has a purple stain added to it.
You can buy IMS, or Industrial Methylated Spirit, which does not have the purple stain. Any of these spirits could be used for making polish, but the finishing spirit is the easiest.
Neither finishing spirit nor IMS will colour the polish whereas the purple dye in normal meths does add a slight tint of purple, which is hardly visible, but not ideal.
To make the recipe, take about 250ml of finishing spirit or IMS and put it in a jar with a stopper. Add to this about 60g of shellac flake, put on the stopper and agitate every hour or so until all the flakes have dissolved. This will probably take a day or so. There will be no harm if you leave it overnight in the workshop without agitating it. What it will not like though are freezing temperatures, so bring all your polishes and glues indoors in wintertime. The same applies to any mixed up water-based stains. Cold and damp are the enemies of polishers.