It is important to have a reasonable grasp of the basic history of furniture when repairing and restoring, as with any art works, to know the development of furniture allows you to look for tell tale signs od deterioration, some times stopping further problems from occurring.
Age can been approximated by various details in design and construction, a knowledge of this will give rise to a more sympathetic restoration and one that can be quicker and easier to perform. If you are aware of the date for different forms of casting materials it will follow that you can have an idea of the age of a cannon or brass telescope, also it will be apparent in the style of an oak chest, done without any glue being used, the approximate age, the cutting of veneers is another example, the reduction and scarcity of exotic woods started to create a market for veneers and the finer the veneer the more could be extracted from the rare source material. The development of the nail and screw, from blacksmith wrought to engineers turnings gave signs for one to assess the age of furniture, this is why a restorer keeps all the old nails and screws that he/she comes across, it is even more necessary now since the availability of the common screw is itself threatened by the philips or posi drive.
A normal way to judge any antique is to pick it up and feel its presence, touching an object can gives instant indication of its material, texture, density and weight, with wood, the colder it may feel can give the impression of more density, heaviness, with visual appearance this can be a sign of more mature timber and more age, the passing of your fingers over the surface will give an idea of the grain's texture, type of wood and its age may again be established when added to the visual inspection, finer grain may be true of a longer growing period and older variety of a species of timber. The fingers will also pick up tell tale signs of previous repairs and changes in polish or laquers.
Knowledge of different timbers can be useful in assessing types of glue that are practical and how certain polishes will take to the surface, it may be necessary to de grease timber or roughen it in order for glue to hold. Antique mahogany very often had virtually one coat of polish or just oil, as a means of enhancing the grain, whilst the Victorian period needed to apply possibly seven or more coats on top of flling the grain with plaster of Paris, the later timber was so open that it needed to be filled and still it would absorb huge amounts of the shellac polish.
It is often the case that your only contact with the origional makers or artists, is with their work, this can, if you wish, have an intimacy , for you will see far more of their marking out, mistakes and style of construction or painting that is solely their own, often you will see the small marks of a chisel or marking gauge, the difference of one maker to another with the shape of a curve, which can also be indicative of age, as is evident in the changing styles that can be seen in front legs of a chair and table legs.
The concept of patina and its association with age is worth remembering when re polishing furniture or cleaning paintings, it is not necessarily good to remove all signs of age but it may well be worth clearing the cloudiness of a shellac polish for the true colour and grain to be appreciated. The refractoriness of a shellac polish, alters with age and when the light can no longer pass through the once transparent layer, it is reflected as a white or bleached look. Some times we may well be valuing the age of an object and not its aesthetics, early veneered pieces where often made to be, by our tastes now, very bright and gaudy but time as tamed them behind a cloudy varnish, we are used to them as pale objects, yet if they are cleaned and the polish is revitalised, there can appear an all too different beast, this has been the start of many arguments in the field of restoration with paintings, for many centuries the restorer was aware of the likely controversy, cleaning would have, so they would clean and then re varnish with a tinted varnish so to not offend the viewer.
The appreciation of art is not always clear and it is shown in a short note about Shelley, whilst touring in Italy. 'travelling in Italy, he arrived at a convent just as the village plumber, glazier and painter(house), was withdrawing his workmen from their task of touching up the old masters,which operation had been included in the contract for renovating the paint and whitewash of the Holy edifice'
A note about fillers, gesso is historically used as a base for paintings, like most things with paintings, the process often then gets passed to furniture, the lifting, relining and transferring of paintings was done far earlier than the use of veneers on furniture but the veneer is glued with the same process as the early artist/ restorer used to remount the ageing canvas. Gesso with animal glue is the base for paintings and often used as a base for furniture that requires a high lacquer finish, plaster of Paris, if mixed and spread over the timber will, once thoroughly dry, take polish on top of it and become transparent as it absorbs the shellac, or can be stained with the whole of the piece before polishing, I often seal timber with a thin coating of animal glue before staining ,so as to limit the stains effect on the changing direction of grain, going darker into the end grain and staying pale on the side of the fibres, this is another point when staining, to slightly roughen the grain in order to get a more even colour. A gesso paste can be made from- white lead, some ochre, oil varnish(siccatif de courfrais) resin and turpentine oil, after drying the gesso must be sanded and when dry, coated with a 10% size solution( animal glue that is diluted so that it is more of the consistency of yoghurt) to prepare the gesso, a small quantity of the glue size is placed o a roughened thick glass palette, alabaster plaster is then added until, after working with a palette knife, it forms a firm, barely malleable paste, care being taken not to arrive at a gesso that does not correspond to the area to be repaired, not to be darker than its surrounding, for light areas white pigment is added and for darker areas, more ochre. Finally some amber varnish is added in small quantities, constantly mixing to form an easily malleable paste. A horn palette knife or spatula is used to firmly and precisely spread the paste into the defective areas, completing the process with a little water to smooth out the final surface, on paintings it will be necessary to wipe the surface with a moist cloth so as to leave the gesso only in the areas of damage and not on any of the paint. This gesso will hardly shrink, should remain firm but porous, its absorbency making it ready to accept further treatment, it is possibly best to let the gesso sit very slightly below the true level so as to allow varnish and paint to be applied up to the true level.
An assistant some years ago remarked, on seeing me use saliva as a vehicle whilst restoring a piece of furniture, that it was highly unprofessional, and when I asked why, he said that most of what he saw me do was not to be found in any of the books that he had read or at the college that he had been . I remarked to him that there can be certain limits to the amount of knowledge he may find in books and that it is always a case of learning first and discarding afterwards, he was welcome to use or discard what ever he saw me do but as my knowledge spanned more than thirty years it was worth considering why I did what I did. Linseed oil, resin, mastic varnish, copal, lavender oil, lye, butter, common wood ash and even soil, where used in the cleaning of paintings, to brighten the paintings, certainly it is not advisable to copy all these but it is worth considering why they might work and where you can use them safely. Spit is a very good cleaner and will dissolve grease and dirt a lot quicker than many propriety cleaners, a lot safer as well, the artist often used egg white as the only means of getting an even glaze to a new painting before it had time for the paint itself to harden, which could be several months to a year, using a varnish too early on would coarse the paint to remain liquid beneath a sealed coating where as the egg would prevent dust and smoke sticking to the fresh oil paint and still allow the oils to dry naturally.
Varnish-middle English vernisshe, from Old French vernis, from Medieval Latin veronix, vernix, sandarac resin, from Medieval Greek verenikē, from Greek Berenikē, Berenice (Benghazi), an ancient city of Cyrenaica.]
Copal, is a slow drying resin, several species of the genus Copaifera, the trees of the Leguminosae family, growing in the tropical regions of the Americas. Deep incisions, canals( similar to those made for extracting latex) are cut into the bark and the balsam flows as does the turpentine of the pine tree, another natural oil ( not to be misconstrued with the white spirit that is sold as a cleaner)
Copal is made up of solid resins and etheral oils, this is similar to resin varnishes, which are a solution of mastic, or Damar resin(resin from India) in oil of turpentine. Genuine copal has a consistency of fatty oil but does not contain oil for the purpose of painting or conservation. When heated in a porcelain bowl, it should not give off any odour of turpentine, and on cooling should be a transparent, brittle resin.
Copal for balsam( para balsam) and the more viscous (maracaibo balsam) differing in the content of etheral oil, the para balsam will change to the maracaibo if left to stand in contact with the air for a long time, or by boiling with water. Copal as low tension properties, and slow drying, turpentine oil is on the other hand has a tendency to evaporate quickly at low temperatures and will have higher tension; copal, heated in a retort over water,will stay virtually unchanged after cooling,while, under the same conditions, conventional resins and that of turpentine will solidify. Contrary to this, linseed and poppy oils are used in the making of oil paint because of their tendency to dry quickly.