10 Decorative Processes
ETCHING
Etching on metal is done with acid. The metals used are usually copper, gilding metal, brass and aluminium. Certain parts of the metal are covered with an acid resisting substance so that the bare parts only are attacked when the work is in the acid. The depth of the etching depends on the strength and the temperature of the acid, the type of metal being etched and the length of time it is exposed to the acid.
Acids
Four parts water to which i part nitric acid is added is often used. This can be dangerous if spilt, also fumes are given off which might cause discomfort. A much more reliable acid is a saturated solution of iron perchloride. (This and all the other materials here mentioned can be bought from the addresses shown at the back of this book.) The saturated solution is made from lumps. No fumes come from this and it is relatively harmless if splashed on to the skin. However, as a precaution against staining, always wash your hands after using it.
Cleaning the Metal
Before any acid resist is applied the metal should be de-greased with fine pumice powder or whiting and water to which a few drops of ammonia have been added. This can be mixed in a saucer and vigorously rubbed on with cotton wool. A good detergent can be used instead of ammonia.
Resists
For convenience we can put these into three groups but there are of course other ways such as dusting with powdered resin etc.
Applied resists. These are applied in the form of cut out shapes of plastic material known by the trade names "Fablon" and "Contact", which have a strong adhesive on one surface. Figure 1 shows a small dish on which a cut shape has been stuck. Be sure that the plastic shape is pressed well down at all the edges.
"Letraset," which is the proprietary name for sheets of transfer letters and motifs which are available in scores of different sizes and type faces, may be used as acid resists. The sheets may be purchased from most artists colourmen and the letters can be stuck on to the metal either in the form of a pattern or as lettering for wall plaques etc. Providing they are well stuck down very accurate lettering can be produced by this means (fig. 2).
Painted Resists. The best known of these is quick drying stopping-out varnish which can be bought or made in the following ways:
(a)
1 part benzine or benzol by volume
1 part asphaltum powder by volume
This must be made in a screwtop jar. The asphaltum powder should soon dissolve. Always screw the lid on after use. If benzine or benzol are not available petrol may be used as a substitute.
(b)
4 oz asphaltum
8 oz turpentine
The asphaltum and turpentine is put in a jar which should be stood in a saucepan of water and heated until thoroughly dissolved.
Stopping-out varnish is painted on to the parts to be protected ; this includes, of course, the back and edges of the work.
Dabbed Resist. This is also known as "dabbed ground" and it can be bought. It is in a small box and is sometimes known as etching ball. Do not buy the "soft ground" which looks the same and is often sold in the same size box. Soft grounds are used by etchers for obtaining the effect of pencil lines on a copper plate. This method does not concern us here.
Solid etching ground can be made as follows:
1 oz beeswax
1 oz asphaltum
1/2 oz Swedish brown pitch
The wax and pitch are melted together in an enamel saucepan and the asphaltum powder gradually added while stirring with a glass rod. The whole is allowed to simmer for 20 minutes more or less (depending on the quality of the pitch) and then poured into lukewarm water and rolled into balls. This mixture is applied to the work with a dabber which is made in the following way. A disc of cardboard about 2" in diameter is cut; a bunch of rag is placed on this and a wad of cotton-wool over all.
A piece of pure silk being placed over the wadding, the bunched ends are held firmly in the left hand and whipped with string beginning next to the fingers and working towards the card. This gradually strains the silk tighter and tighter over the cotton-wool (which must be worked into shape) so that no folds are left over the front surface of the pad (fig. 3).
The work is warmed on a hot plate to such a heat that the etching ball will start to smear on the article as it is stroked across.
At this heat it can just be held in the fingers. With the dabber, carefully start dabbing the smears of ground evenly over the article. Do not overheat! Ideally it should feel tacky when being dabbed. The thickness of the ground should be such that it is slightly translucent. When the article is evenly covered it should be returned to the hot plate and moved about until the whole surface becomes glazed. It can then be stood against a vertical surface, face inwards, so that the dust will not settle on the surface while it is cooling.
The design is transferred from tracing paper on to the metal using red transfer paper or carbon paper (fig. 4). When the design is transferred on to the ground it can be scratched through with a needle or for broad areas a small knife blade. Remember that clean sharp edges will remain sharp during etching, but ragged lines will become more ragged.
Ideas for designs can often be developed from natural forms. Figure 4A shows six sketches taken from nature and from much enlarged photographs of details of natural forms. However, natural forms should not be copied slavishly but used as a basis for designs.
Use of Acid
When using nitric acid the work is either immersed in the acid which can be in a porcelain, glass or plastic container, or if the work piece to be etched is deep enough, such as a bowl, the acid can be poured in. Nitric acid works vigorously forming bubbles which should be brushed off with a feather every few minutes whilst the work is being etched. If this is not done ragged lines can result.
Work done with iron perchloride should be bitten either face down or on edge in the acid because the sediment which forms tends to "choke" the effect of the acid if done face up. If the work is such that it must be done face up then the acid must be agitated with a feather every few minutes to remove the sediment.
To see how deep the acid is biting, remove the work from the acid (avoid putting your fingers either in the iron perchloride or the nitric acid) and wash under a tap. Dab with a soft cloth to dry. If it has to be returned to the acid check to see if the acid resist, front and back, needs "touching in" with stopping out varnish. This is sometimes necessary because small "pin holes" can develop in the ground.
Removal of the Resist
The plastic applied sheets can of course be peeled off. The other grounds can be removed with a soft cloth and turpentine.
Never buff etched work as this ruins the characteristic sharp edges of the etching. Use a soft hand brush with metal polish or whiting and methylated spirit. Finish with a soft polishing cloth.
ENAMELLING
Enamelling is the process of fusing glass on to a metallic base. The glass is usually crushed and it can be coloured or clear, opaque or translucent. The metallic base, for school work, is almost always copper, although gold, silver and steel can be enamelled.
The art of enamelling has been known for centuries. The earliest examples were found in Cyprus in the form of enamelled gold rings dating back to the thirteenth century B.C.
Enamelling is a fascinating art and there are many different techniques. The interested reader should refer to the bibliography at the back of this book. However, here is a simple enamelling technique which can be used readily in school with the minimum of equipment.
Requirements:
small slightly hollowed copper dishes (to be enamelled)
pumice powder and ammonia
powdered enamels
fine mesh coffee strainer
gum tragacanth made up in liquid form (from any chemist)
a water colour brush
heavy gauge wire mesh
small cover bent from 22 SWG stainless steel
blow torch
brazing hearth and fire bricks
carborundum slip 6" X 1/2" X 1/2".
Method: Clean the copper dish as for etching. Rinse thoroughly and brush on gum tragacanth. Lay the dish on a piece of clean white paper and sieve your chosen enamel evenly over the surface (fig. 5). Do not leave any bare patches and be sure the enamel goes right to the edge of the dish. The layer of enamel should be about 1/32" thick. Carefully place the dish on the wire mesh for firing (fig. 6). Any enamel left on the paper after the dish is removed should be returned to the enamel container.
Start firing with a very small soft flame to dry off the enamel, then heat evenly to cherry red with a large intense flame. When the enamel has fused allow to cool slowly on the mesh.
The first coat is now on the dish. Paint with gum tragacanth as before and stick into place the cut-out paper shapes (fig. 7). The second layer of enamel is then dusted over the dish as before. With a pair of tweezers carefully remove the cut-out shape thus leaving the original fired surface free of powdered enamel. Fire as before. Now we have a dish in two colours. This could be quite satisfactory at this stage, but if further dustings, possibly with transparent colours, are required they can be done.
Finish the dish by carefully stoning the edge with the carborundum slip and water. Clean the back with fine emery cloth. Take care not to distort the work or the enamel will fracture.
ENGRAVING
There are many different tools available for engraving, but the one most used is the square graver (fig. 8). It should fit the hand as in figure 9. This graver can be made from 1/8 " square silver steel. The tang is filed and bent and the tip hardened and tempered to light straw—do not go beyond light straw. The point is stoned as in figure 10, first on a medium India stone and then finished with a circular motion on an Arkansas stone (fig. 11).
Professional engravers often test the sharpness of the graver by touching the point at an angle against the thumb nail. If the tip catches and does not slide, it is sharp.
Transferring the Design to be Engraved
1. For highly polished surfaces. Trace the drawing. Rub Russian tallow or plasticine on the back of the tracing paper, then lay it on the article to be engraved and with a blunt scriber transfer the drawing. This will leave a copy of the design on the metal. The fine lines of Russian tallow or plasticine on the metal can then be gone over with a scriber to prevent them from being rubbed off. The metal is now ready to he, engraved.
2. Rub plasticine or beeswax all over the area to be engraved and then dust with whiting. Now with a sharp pointed piece of hardwood draw the design. This can be corrected as many times as you wish. Finally, lightly scratch in with a scriber and engrave.
3. To reverse the design. Trace the design on transparent acetate sheet (0.010" thick works well) with a scriber. Rub pencil lead into the tracing. Now lay this (in reverse) on the metal which has been rubbed with beeswax. Rub the back of the acetate sheet with a burnisher or the back of a spoon. The design will now be transferred on the work in reverse. Go over this lightly with a scriber before engraving.
The work to be engraved is usually rested on a sand bag and the engraving tool point is pushed gently along the marked-out lines—lightly for the first cut—turning the work and not the tool at the curves. Make the lines deeper where required by re-entering with the engraving tool and taking more cuts. Figure 11A (designed and engraved by the author) shows an example of engraving.
REPOUSSÉ
This is a method of decorating a sheet metal article by means of raised or indented motifs, made with punches (these are blunt and never cut the metal) whilst set in pitch. The tools and materials used are:
Punches made from silver steel (fig. 12)
Repoussé or chasing hammer (fig. 13)
Pitch
Cast iron pitch bowl and leather ring stand (fig. 14) (A shallow metal tray will suffice for most school work.)
Blow pipe
The Pitch
This must be tough enough to support the work and yet resilient enough to allow the metal to be driven in by the punches.
A good recipe for pitch is as follows:
7 lb Swedish pitch
7 lb Plaster of Paris
1/2lb tallow
The pitch is melted, the tallow added and the plaster stirred in a little at a time. Before the work is set into this pitch it should be lightly greased to facilitate cleaning after removal from the pitch.
Method
Assume we are starting with a flat piece of 22 SWG (this is probably the best gauge for repoussé work) soft copper. Have the pitch in a 1" deep metal tray about 1" larger all round than the workpiece. Warm the surface of the pitch with a soft flame and press the copper sheet on to the surface so that the pitch just comes over the edges. Be sure there are no air bubbles under the copper. Allow to cool. Trace the design on the copper using carbon paper.
Remove the carbon paper and lightly scratch through the design with a scriber. Hold the tracing punch in the left hand at such an angle to the work that when it is struck repeatedly with the chasing hammer it moves towards you making a smooth indented line following the lines of your traced design. A little practice is necessary before this can be achieved.
When you have finished with the tracing punch, warm the surface of the work with the blowpipe and remove. Clean with rag and paraffin. The design now shows in the form of raised lines on the back. Set the work in the pitch as before but face down this time. The parts to be embossed are now punched down with suitably shaped punches. The work may be reversed as many times as required until the desired effect is obtained. Figure 15 shows a part sectioned piece of repoussé work. Figure 14 shows the cover of a circular box set in pitch.
Cast iron is used for the pitch bowl because it is heavy and withstands the effect of the punching.
CHASING
Chasing is often not unlike repoussé work in appearance and it is done with similar punches, but it is done from the outside of the work—not from both sides. Castings can be chased with fine corrugated punches which produce a matt surface and also close up any small blow holes.
EMBOSSING
This is the term given to work which is punched entirely from the back. Often a snarling iron (fig. 5, Chapter 9) is used for this purpose.
BENT WIRE DECORATION
The base of vessels can be decorated by the soldering on of wire bent to form a repeating pattern. Figure 16 shows soft brass wire being formed on a jig. The jig can be made from hardwood using sawn-off nails which have been carefully spaced and rounded at the top to allow the wire to be removed easily. For more precise work the jig can be made of brass with accurately spaced steel pins.
COLOURING AND FINISHING METAL
Anodising or Anodic Oxidation
This is the process whereby the oxide film on aluminium is thickened electrolytically. This thick film increases the resistance to corrosion and it can also be dyed.
The aluminium is first de-greased. The most common electrolyte is dilute sulphuric acid which is in a lead lined bath (cathode). The aluminium article becomes the anode. After 30 to 60 minutes the article should be taken out, thoroughly rinsed and dyed if required.
The Aluminium Development Association publishes a booklet for schools on this subject in which full instructions are given.
Colouring of Copper, Brass and Gilding Metal
There are various colouring agents which can be bought and many recipes available, particularly in some of the older books. However, many of these recipes are not reliable. A good mixture which will colour these metals chemically can be made as follows:
photographer's (plain) hypo 10 parts by weight
sugar of lead (lead acetate) 1 part by weight
citric acid 1 part by weight
water 40 parts by weight
Clean and de-grease the work to be coloured. Dip in the mixture. The metal will slowly change colour, first golden, then rose, blue, grey-green to grey. It will take 5 to 10 minutes to reach the final colour. If the mixture is kept moving the colour will be more uniform. When the desired colour is obtained the work should be lacquered after it has been washed and dried.
Blackening Steel
Small steel items can be blackened by heating gently until hot enough to scorch a piece of wool blanket which is rubbed on the surface. The blackened article should then be lightly oiled. This method is particularly good for small items but beware of the nasty smell!
Coat the steel with mineral oil and burn off on the brazing hearth until black.
Heat the steel and rub with saltpetre.
Blueing Steel
Polish the work with fine emery cloth avoiding oil and grease. Heat to blue as for tempering, quench in paraffin and polish with beeswax.
Burnishing
This is the process of polishing soft metals, such as gold, silver and copper, by rubbing with a hard smooth tool made from agate, bloodstone or hardened steel. Various shaped burnishing tools are used and soapy water is a popular lubricant. The polish obtained by an expert can be remarkable. The action is one of pressing down the minute surface irregularities so that the surface becomes consolidated.
Mottling
This is a finish obtained on small pieces of engineering work by means of carborundum paste and 1/4 " diameter dowel rod held in a drilling machine chuck. As the rod spins the end is pressed against the work causing the carborundum to make polished circles on the surface. If the circles are kept in neat rows this type of finish can enhance the surface, but it should be used with restraint.
Electroplating
This is a process whereby a thin layer of metal is deposited on another by means of an electric current passing through a solution of metallic salts.
It is possible by this means to deposit gold, silver, nickel, chromium, cadmium and copper. There is a mistaken belief among some students that plating will hide blemishes in their work. It will not: plating emphasises blemishes.
Lacquering
Lacquering is done to protect metals from the atmosphere. It is a clear varnish (often with an amyl-acetate base) to which colouring matter is sometimes added. It is applied to thoroughly cleaned metal by brushing, dipping or spraying. In school, lacquer is usually applied with a brush; for best results a good quality camel hair brush should be used. Care must be taken to ensure that every part of the surface is covered, otherwise the uncovered parts will oxidise and show later as streaks. Badly applied lacquer must be removed using the solvent recommended by the manufacturer and then re-lacqucred.
Painting
Iron and steel work which is to be exposed to the weather should be protected, unless it is stainless steel. Paint is the commonest protector. Before painting the work should be free of rust, oil or grease.
The paint may be brushed or sprayed. Several thin coats are better than one thick coat. Cracks often occur on a thick layer of paint because the outside of the paint dries first and contracts, whilst the layer of paint nearest the metal is still soft and cannot "anchor" the top layer to the metal.
If castings are to be painted any blow holes must be filled with a filler which suits the paint to be used and rubbed down between coats of paint with wet and dry emery paper.
Plastic Coating
This gives a durable and attractive finish to metal articles such as draining board racks or shoe racks which are never quite satisfactory when painted.
The article is heated in an oven to about 200°C then covered with a layer of a proprietary powder which is then fused on to the metal by reheating. Take care to follow the maker's instructions.
