TIMES AND TESTS
electrical units and ohms law It is useful to understand something about the units in which electricity is measured to appreciate the discussion of how long is need to galv-etch plates. If you are already familiar with all this, skip this paragraph. The 'strength' of electricity is called the potential difference and is measured in Volts, and it is usually called the voltage. You can think of it like the pressure of water in a pipe. The tap may be shut but the pressure is there all the same. The 'rate' at which the electricity is passing.is the current and is measured in Amperes, and as the name 'current' implies, you can think of it as the rate of flow of water in a pipe only when the tap is opened. The 'power' of the electricity that flows is the potential difference (volts) multiplied by the current (amps), and is measured in Watts. The power or 'strength' of water flowing out of the tap depends on the pressure in the pipe and the rate it is allowed to flow by the tap. The quantity of electricity is the power multiplied by the time and is measured in Watt hours. Using the water analogy, when the tap is opened, a bucket can be filled at a certain rate, say a litre per minute, and the quantity is the total volume of water that has flowed into it after a given time. When a current flows in an electrical circuit it meets a resistance measured in Ohms, which reduces its rate of flow. In that it is like the tap, which increases its resistance as it is gradually shut, slowing down the rate of flow of the water. As an electrical resistance decreases, the potential difference, the voltage decreases and the current, the amperage, increases, and if the resistance increases, the voltage increases and the amperage decreases. The ratio between volts, amps and ohms is very simply expressed by Ohms Law. Ohms = Volts/Amps. The current flowing between copper plates in an electrolyte is related to the voltage of the power source and the resistance provided by the electrolyte. If the plates are very close together the electricity has little resistance to overcome the flow between the plates, and for a given voltage the amperage will be high. On the other hand, if the copper sulphate is diluted and therefore offers a greater resistance (because the concentration of ions is lower in the solution), then the amperage will be lower. So a voltage of 4 volts meeting a resistance of 2 ohms will cause a current of 2 amps to flow. (.A = V/O). I hope that this simple explanation of principles and units will make the subsequent discussion of current, voltage, resistance simpler to understand and demystify the subject a little. TOP The time required to etch to a given depth can be simply calculated if you have a means of measuring the amperage and voltage that the power supply is delivering when the plate is in place and the system is switched on. These are dependant on the resistance provided by the area of bared metal on the plate, the strength of the electrolyte, the distance between plate and grid and to a small extent, the temperature. But all these factors can be taken into account by 'calibrating' your setup by making a special plate about the size of the grid or cathode plate, with an area of bare metal measuring 100 square centimeters (15.5 square inches). This should be in the form of several rectangles distributed over the area of the plate. Protect the back with a self adhesive plastic sheet (holding the contact strip), place the plate in the electrolyte and switch on. Leave the current on for a minute to let the amperage and voltage settle down and note the readings on voltmeter and ammeter. If you have a regulated power supply, set the voltage at 1.0 and take an ammeter reading, then set it at 1.5 volts and note the amperage and so on up to about 3.0 volts. You can then calculate the resistance R of the system by dividing the voltage by the amperage. You may get slightly differing results for R at different voltages, but if they do not differ greatly, take the average and note it down for future reference.
Then when you have a plate with lines drawn in a hard ground, prepared for etching, in the same tray or tank, with the same electrolyte and distance, you can calculate how long to give it to bite the lines very lightly or to bite them quite strongly. The table below gives a series of numbers F for copper, zinc and steel (iron) plates, which you multiply by the resistance R you obtained by calibration and then divide by the voltage that you are using (if you have a regulated power supply on which you can set the voltage). If your power supply is unregulated - a battery charger or rechargeable battery pack, then you must put the plate in and turn on for long enough to read the voltage (see discussion later in this section). T = F x R/V ; where T = time in minutes; R = resistance obtained by calibration with 100 sq.cms., and V = volts; F is obtained from the table below :
Simply, the time taken to etch a plate depends on the voltage - the higher the voltage, the less time it will take and vice-versa. Each printmaker has his own preferences about the depth of bite required, so the values for F are a guide based on my own ideas of the meaning of 'lightly etched' etc. It is harder to give a method for calculating the time required for deep etch or open bite, as these are more subjective and depend on an individual's expressive intentions. Note that this method of calculation does not depend on the size of the plate but is valid for any plate size within limits (within the limits of a given tray/grid/electrolyte calibration test). making a test plate If you are wanting to etch a plate with open bite or any kinds of treatment that do not fall into the category of needled lines, make a test plate of the size you will most often use, with a typical range of the kinds of marks you use. Calculate the time for a light etch as above, halve it and then galv-etch it. Take it out, dry it and stop out a strip, and put it back for the same number of minutes, and so on in about 8 - 10 steps, noting exactly what you were doing. Then clean it and proof it, and keep it as a guide to the times to achieve the results you want. For very deep etch the times for each step should be doubled or tripled. Note that very small test plates can give a very misleading idea of how larger plates will behave, because the current intensity could be so great that grounds will be lifted and the bite will be irregular and results discouraging. So trials on small plates must be at lower voltage and current than you need use for larger plates. Larger plates in a tray with large exposed area of open bite may overload a small power supply, and if that happens, there are a number of different options:
the effect of voltage and amperage There has been some discussion of the importance of only using low voltage and that the use of a low voltage was an original discovery in 1990. My research has shown that the earliest uses of Electro-Etching in the 1850's involved the use of a low voltage (about 1 volt) because that was the practical limit of the Daniell's or Smee's cells that were used to supply the current, and that artists using electrolytic methods subsequently often used a low voltage. There is no doubt that a low voltage can be used, and there are a few circumstances where it is an advantage. But as can be seen from the tables below, the voltage output at the electrodes is not the same as the voltage set on an unregulated power supply like a battery charger, a battery or produced by a solar photovoltaic array, but is reduced when the resistance of the plate is low, and is reduced still further by the addition of a resistance in series. In fact it is not voltage that is important, but amperage that determines the quality of bite. The ratio of voltage to amperage is constant for a given resistance, and reducing the voltage reduces the amperage proportionally. If the time calculated to deep etch a plate at low voltage is considered too long it can be increased, and the amperage intensity per unit area will increase proportionally, but the type of resist will need to be more robust. The tables below are a guide to voltage and amperage at various switch settings, using the standard equipment shown earlier. The first table is for a small needled plate, and the second, a plate with a large area of open bite. These represent the extremes in normal practice. Table of voltages and amperages recorded with an unregulated 6 volt supply - for a needled copper plate 275 sq. cm. (44 sq.in.) in 1:4 copper sulphate, with 6 cm. (2.5") between plate and grid.
Table of voltages and amperages recorded with an unregulated 6 volt supply - for a large copper plate with 625 sq. cm. (100 sq. inches.) of exposed metal in 1:4 copper sulphate, with 6 cm. (2.5") between plate and grid.
The use of a very low amperage and voltage is advisable only if one is using a fragile resist on a very small plate. After a little experience the voltage required to produce an acceptable amperage for a given plate size and treatment can be found and noted. For practical purposes most useful electrolytic action from a printmaker's point of view takes place in the range of about 10 amps down to 1 amp, and a creative printmaker must use the level that is appropriate to her/his work:
For most normal purposes the time required for etching at the lowest amperage would be far too long, and most printmakers would prefer to have the choice and be able to switch to a higher voltage setting or take out the resistance. In doing the tests is is important to use a plate of the use you will normally use. Very small test plates will produce very misleading results. It is always a good idea to start with a very short bite and take out the plate, rinse it and examine it carefully with a magnifying glass for pinholes or scratches which should show clear and bright in a dark ground, or for the breakup of the ground. The galv-etch process is unforgiving about carelessly grounded or ill-treated plates, which is another reason why the insulating relief ink grounding method is superior to traditional smoked ground, as it is less liable to pinholes. Stop any faults and then continue with the process. If the ground shows signs of breaking up, then the amperage must be reduced as described above. A time-switch is a very useful accessory - I use a simple off-the-shelf mains electronic switch with a remote control (illustrated below ) which controls the mains supply to the battery charger. If you are using a direct current supply like a battery pack or a solar cell array, then you can find electric time switches that operate an electric relay on the direct current output. (TOP)
galv-etching very large plates A very large plate will have a much lower resistance than a smaller plate, and with your power supply set at the same voltage, a much higher amperage will be required. Your power supply may not be capable of supplying the required amperage, and will cut out or blow the fuse. The safest solution in that case is to lower the voltage, and accept that the time taken will be much longer - with the lowest voltage, several hours for a deep etch. If the nature of your image allows you to work on a part of a large plate at a time, then the galv-on semi-dry method will provide a simple solution without needing a a heavy duty power supply. If you are working with large plates or with very large areas of open bite for relief or viscosity printing, and where you do not want the galv-etch to take a very long time by reducing the voltage and amperage considerably as previously described, you may have to use a very heavy duty battery charger - 6/12 volts with a capacity of 20 ohms or more, and use a very strong resist. If your plate is very large, it is more practical to use a deep horizontal tray with a grid cathode. But the grid cathode frame must be metal heavy enough to prevent it from sagging in the middle, and made of heavy strip metal rather than wire and joints should be mechanical rather than soldered - that is, passed through holes in the frame and bent to hold. The electrolyte may have to be further diluted to increase the resistance and keep the load on the supply within limits. Plating or galvanoplasty should be done at a voltages and currents that is appropriate to the size of plate, and it is impossible to generalise, but read the section on safety precautions first. The only way to determine what amperage will work is by experiment with the size of plate that will be used. What works fine on a small plate will not have the same effect on a plate twice the surface area.
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