The basis of electro-etching:
a simplified explanation CONTENT | SEARCH
To etch the copper plate, the two plates are held vertically in the electrolyte, facing one another and separated by about 50 mm. The battery and switch are connected between the two plates (electrodes), with the positive terminal attached to the plate which is to be etched and the negative terminal to the other copper plate; when the switch is closed electric current flows and etching takes place.
This article presents some of the underlying ideas and terminology connected with electro-etching. It is not crucial to understand these principles in order to produce successful etchings. However, we believe that by presenting a simplified explanation, it may remove some of the mystery of the technique and assist in encouraging its wider use.
Part 1 BACKGROUND
Following an introductory overview, some physical and chemical principles relevant to electro-etching are introduced. No prior knowledge is assumed and no mathematics is involved.
Basic kit for 'Home' electro-etching:
1 - power supply based on using rechargeable batteries
2 - inexpensive multi-meter to monitor electric current
3 - polymer containers holding electrolyte
4 - electrodes and connecting wire
5 - plates to be etched
6 - tape and clips for masking and attaching plates to anode electrode
7 - polymer container for holding equipment during etching
8 - dust mask, gloves, and safety glasses
2. An overview of electro-etching
Essentially, the etching of the copper plate mentioned above is a result of the following electro-chemical process:
• Where the needled lines have broken through the ground, the surface of the copper plate
is in contact with the liquid electrolyte;
• At these points of contact – the metal/liquid interface -and only at these points,
the electrical energy obtained from the battery
enables some of the copper atoms to give up their outer shell electrons to the copper plate;
• These copper atoms thereby become positively charged copper ions Cu++ which
break away from the surface of the copper to become part of the liquid electrolyte;
• The amount of copper atoms which dissolve is proportional to the electric current flowing
and the length of time the circuit is switched on.
The ionization and transfer of the copper atoms from the plate to the neighbouring liquid, described as “dissolution” by electro-chemists, corrosion by material scientists, is more easily recognized by us as etching.
The key feature of electro-etching has been introduced: the atoms at the surface of a metal plate may give up some of their electrons to the external electric circuit to become positive ions which “dissolve” into the adjacent electrolyte. It now remains to fill in some of the details.
3. The material world – atoms and electrons
3.1 What we are made of – atoms; protons and electrons
Everything in the world is made from a combination of only a hundred or so substances, so-called elements, the majority of which are metals. The smallest amount of an element that can exist is an atom (from the Greek atomos “uncuttable”).
An atom is formed from just three sub-atomic particles, the proton, neutron and electron. The simplest atom belongs to the element hydrogen: it has a single electron orbiting around a nucleus consisting of a single proton. The proton possesses a “positive” electric charge while the electron has an equal but opposite “negative” charge so overall there is no excess charge –the atom is electro-neutral.
3.2 The motion of electrons –orbits and shells
One way of visualizing an atom uses a “solar system model”: the nucleus is the Sun and the electrons are the planets moving around the Sun in various orbits but with the difference that in a given orbit there can be more than one planet (electron). The permissible orbits and the number of electrons which can “occupy” a particular orbit or “shell” are governed by the rules of quantum physics.
Copper plate after electro-etching in copper sulphate; ground (candle wax) still on plate; dark areas: natural “electro-tint”
3.3 Outer sub-shells determine chemical properties
The chemical properties of an atom and the way it may interact and combine with other atoms to form molecules are largely determined by the number of electrons occupying the outer shells/sub-shells where the influence of the nucleus is less. Some elements, for example the gas argon, have atoms which have just the right number of electrons to completely fill one or several shells/sub-shells to give a stable, low energy structure, making them inert and unable to react with other atoms.
Figure 1: Rutherford “solar system model” of the atom, courtesy of Wikipedia article “Atom”
4.1 A basic property of metals
Characteristically, for metals the outer sub-shells will be occupied by only a few electrons, much less than the shell is able to accommodate. By contrast for many non-metals the outer sub-shells will be almost full of electrons.
4.2 A simple picture for the outer electrons within a metal – electric current
In a piece of metal there will be many millions of “outer” electrons which are essentially free to move within the open lattice, formed by the comparatively massive, immobile nuclei, and held together by the residue attractive electric forces between the “roaming” outer electrons and the nuclei. This may be pictured as a fixed array of buoys – the nuclei – bobbing on a “sea of electrons”. The passage of an electric current through a metal is really the flow of these electrons.
4.3 Sharing electrons and the formation of ions
Metal atoms would be more stable and “happier” if they had a structure similar to argon, that is, fully occupied shells/subshells. They can achieve this under the “right conditions” by donating their electrons in the outer sub-shells to a non-metal which correspondingly would like to gain the same number of electrons to complete its outer sub-shell so that it also has an argon-like structure.
The atom of metallic sodium (Na) has its inner shells full leaving just one electron in its third shell - capable of holding eighteen electrons. The atom of the non-metal chlorine (Cl) again has its inner shells full, leaving seven electrons occupying the third shell, one electron short of filling two of its sub-shells.
By transferring an electron from the sodium atom to the chlorine atom both achieve complete sub-shells. The atoms Na and Cl become positively and negatively charged ions Na+ and Cl -.The electric force of attraction binds these ions together and leads to a lattice type structure incorporating huge numbers of atoms to give the familiar crystals of common salt.
A metal atom, in transferring electrons to another atom ceases to be a neutral atom - it has excess positive electric charge because there are more protons in the nucleus than there are orbiting electrons – it has become a positive ion. Similarly, the non-metal atom which has received the electrons now has excess negative charge, it has become a negative ion. The positive and negative ions thus formed become bound to one another through the attractive electric forces –ionic bonding.
4.4 Metals for etching and their ions
The electron distribution for atoms of aluminium, zinc, iron, copper –the metals of interest for etching -are such that the following ions would be expected if electrons in the outer shells can be lost to form a more stable ion:
5.1 Combining atoms - salts
Test print: line drawing, copper plate, candle wax ground,
electro-etch in copper sulphate; after Rembrandt
Elements seldom exist as single atoms, rather they combine to form compounds some of which consist of thousands of atoms. Of particular relevance for us are the compounds belonging to the class called salts, for example sodium chloride NaCl, common salt, copper sulphate CuSO4 and iron sulphate FeSO4 - each contains a metal atom and a sulphate molecule held together by ionic bonding. These salts are dissolved in water to form the liquid electrolytes for electro-etching.
The weight of a mole of atoms or molecules in grams is known as the gram atomic or molecular weight. A molar solution is one in which the gram atomic or molecular weight of a substance is dissolved in one litre of water, written as 1M solution.
5.2 Dissolving salts in water: electrolytes for electro-etching; electro-neutrality
Salts dissolve in water because the water molecules are able to break up the ionic lattice into positive and negative ions. The number of negative and positive charges is equal so the solution as a whole is 'electro-neutral'- zero net charge.
Test print showing different depths of natural “electro-tint” obtained by masking out copper plate with nail varnish; electro-etch with copper sulphate
Test print: tone obtained with conventional aqua-tint; electro-etch, copper plate, copper sulphate electrolyte; masking out with candle wax
Test print: creating tone with cross hatching; electro-etch, copper plate,copper sulphate electrolyte; candle wax ground; after Cartier-Bresson
Test print: texture obtained by pushing fabric into the candle wax ground;
electro-etching, copper plate
Stencil made by electro-etching through thin sheets of coated aluminium;
sodium carbonate electrolyte
Standard reduction potentials in water solutions at 25⁰C; concentration of solutions 1 mole per litre (1M).
Copper plate after electro-etching in sodium
Evolution of chlorine:
Evolution of oxygen:
Evolution of hydrogen:
See also: “Electro–etching Made Easy” under
etching innovation on this website.
A.) Etching of the anode plate dominates so that the formation of chlorine or oxygen and their effects are negligible.
B.) The formation of chlorine molecules and their escape from the anode in gaseous form, predominates the process.
C) Chlorine molecules are produced by the electrochemical reaction but remain within the electrolyte.
mg/litre sodium hypochlorite
Titanium with DSA coating
Note added December 2014. Recently Cedric Green has added an appendix to his website related to the issue of sodium chloride. We have carried out some electrolysis experiments using aluminium electrodes and sodium chloride solution as the electrolyte and for the conditions studied did not detect any chlorine. For more information see an updated version of our article atEnglish/publications/articles