Philip Quanjer, Else Quanjer
Transferring a design onto metal using photoresist
Occasionally one is at a loss how to transfer a complicated drawing or picture to metal, such as silver or copper. There are various options:
- At one time a recommended procedure was to transfer the drawing using graphite paper; in the kiln graphite burns without leaving a trace, so the method is in principle quite attractive. However, we have been unable to obtain graphite paper anywhere.
- If the objective is to use the design for etching, obviously the use of PnP blue is an option (PnP is shorthand for Press-’n-Peel). These are sheets onto which one can transfer a design (for our purposes it needs to be mirrored, i.e. flipped horizontally) using a laser printer or copying machine. The drawing is subsequently transferred to the metal using an iron heated to about 150 ºC (300 ºF). The 'unexposed' part of the PnP Blue can be peeled off, the 'exposed' part functions as a resist, protecting the metal against etching substances. PnP is extensively used in the USA, we had great difficulty in finding a distributor in the Netherlands. We might have obtained it from somewhere else, e.g. from Vitrum Signum (now Diatherm & Ancillary Equipment, Gresham Works, Mornington Road, North Chingford, London E4 7DR, tel/fax +44-20-8524-9546), an address well known to enamellers.
However, we used photoresist to this end. This material has been in use for decades in making electronic prints, and more recently for making moulds used to press CD-ROMs. We describe two applications. The first application is to etch a basse-taille relief in a silver plaque, and subsequently enamel the plaque with a transparent enamel. This works well, but you have to pay attention to some details. The following lists the different steps (an overview is available elsewhere on this site).
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| After exposure and development photoresist only remains on the metal in the areas which are black in this picture. The white areas will provide a basse-taille relief after etching. |
- See to it that the drawing or design is made in pure black and white; shades of grey and colours are useless for these purposes.
- Transform the design into a negative (black turns white and vice versa) flipped horizontally. This is easily done by computer using software designed to manipulate bitmaps (such as Photoshop, Photopaint, etc.)
- Using a laser printer print this onto an appropriate transparent sheet, or use an ink printer with an appropriate transparent sheet. You can also make a drawing and go to a copy centre to transfer it to a transparent sheet. Black should really be black on the sheet!
- Carefully clean the silver (or copper) as you would do normally (it should be absolutely grease free), and put it face up on a dust-free old newspaper.
- From now on keep the environment painstakingly dust-free. Avoid direct sun light (the sun emits UV radiation), work in subdued light.
- Hold a can with photoresist (Positiv 20, available from some electronics retail shops, but do pay attention to the expiry date!; ask Kontakt Chemie, CRC Industries for a dealer in your environment) at approximately 20 cm (8 ") from the metal and spray the photoresist, creating an even and not too thin film by applying the spray in serpentine lines starting from the upper left hand corner. It is important that the metal is flat and horizontal, otherwise you will not be able to make an even coating.
- Leave the film to dry. As there is always some fine dust around that will settle on the coating, we first cover the metal and coating for about 30 min. with a plastic box with an opening on one side, such as the one secretaries use to store incoming and outgoing mail. First we make the inside of the box dust-free with a moist towel.
- Before continuing, first ascertain that the coating is smooth and even. If it is irregular and quite thin at places, then remove the photoresist with acetone (nail polish remover), clean the metal surface thoroughly and apply the photoresist again.
- If the coating is smooth and even and free of dust, dry it in the kiln for 15 min. at 70 °C (160 ºF).
- Let the material cool down in subdued light. Put the sheet with the negative face down on the metal with photoresist and make sure that the two are in close contact, for example by pressing with a sheet of glass. Close contact is thus achieved only if the metal is very flat. In case the metal is not flat close apposition can be achieved by putting metal and negative in a thin plastic sheet to which a slight vacuum is applied.
- Expose the photoresist through the negative with UV light. We use a Model Master; it is fairly costly, and we have noticed that electronics retailers sell UV units that are much cheaper. If you are a handy person, you can easily make a unit yourself. It depends on the type of UV lamp, the distance between lamp and film, how long to expose; with our exposure unit about 70 seconds.
- Develop the image in 7 gram caustic soda (NaOH) to 1 litre of cold water, or in Seno 4007 UniversalEntwickler (or the newer Seno 4006), both available from electronics retail shops. The exposed photoresist is removed in this bath and shows as blank copper or silver amid a brownish-bluish film.
- Rinse under a running tap, then gently dry it by dabbing with a towel or tissue.
- Carefully cover the metal that should not be etched (such as the back and sides of the object) with grey or black duct tape. Remember, areas covered with photoresist are protected against chemical or galvanic etching. If your Positiv 20 canister is past the expiry date, you can still use it as a resist, or you can use lacquer used as etching ground.
- We etch copper with ferric chloride. Silver is etched electrolytically; we use 3% HNO3 (nitric acid), a silver electrode, a current of 20-25 mA from a universal battery charger (Voltcraft Unilader).
- After etching the photoresist is removed with acetone, and the silver carefully cleaned so that it can be enamelled. If you wish to have basse-taille with a relief at varying depths, you will have to repeat the process with different negatives for parts that need to be etched more deeply or superficially; properly aligning the negative at each turn, however, is definitely a challenge.