Iron/HCl method This method is a bit more costly than electrolysis, but it's faster. Here we'll first dissolve metallic iron in a strong acid like HCl (or H2SO4) and then precipitate iron hydroxide with NaOH, to process it further. Since some iron alloys, (like cast iron) can contain carbon and silicone (not soluble in the acid), these can be easily separated by filtration when iron have been dissolved. This way, purer rust can be obtained from quite impure cast iron. These impurities are more tedious to remove with electrolysis. Procedure Materials: Steel-wool or nails (Fe) Hydrochloric acid (HCl), 10% Sodium or potassium hydroxide (NaOH or KOH) Here’s 260 ml 10% HCl in 500 ml baker and 22 g of steel-wool: Dissolve >20g of steel-wool in 260 ml 10% HCl (take steel-wool in excess). When major part dissolves, heat solution to boiling. You may add more steel-wool if everything dissolves. Note, above procedures have to be done outside since some HCl will be evolved. Iron dissolving: Fe + 2HCl ==> FeCl2 + H2 Chill liquid to room temperature and filter. Add excess of NaOH solution (>47 g NaOH dissolved in 100 ml water) to obtained liquid. Iron hydroxides will precipitate and first there will form amorphous white mass. Stir it for 10 minutes, there will be colour changes, from white to dark-green. FeCl2 + 2NaOH ==> Fe(OH)2 + 2NaCl Pour the goo into 3-5 l water, stir, and let solid settle on bottom. Decant (pour off) clear liquid and repeat this 2 times with another 3-5 l water. This decanting procedure should remove majority of NaOH, which will make it possible to filter without problems. Filter, and wash it on filter with water, dry. It will change colour along drying to brown because Fe(OH)2 will convert to FeO(OH) on contact with air. Though experimentally it seems that drying at room temperature doesn't convert all Fe(OH)2, probably because oxidation proceeds only on surface of particles. 4Fe(OH)2 + O2 ==> 4FeO(OH) + 2H2O Powder dried "cake" and heat it in an oven at 250-300oC in 1-2 hours. Along heating, reaction above will go to completion and brown FeO(OH) will convert to red-brown Fe2O3, which is rust. Heating dehydrates FeO(OH) by following reaction (approx. 10% loss in weight): 2FeO(OH) ==> Fe2O3 + H2O Actually, content of final product can vary a bit, depending on temperature during dehydration. But it's not that big concern if you're going to use it in thermite. Actually, for thermite, it would be sufficient to hold rust in oven for 1-2 hours at 100oC. This is enough to remove all trapped water and convert most Fe(OH)2 to FeO(OH). This experiment gave 34 g rust, background is a bit red since I tried to adjust color of rust to it's original level: Additionally, instead of HCl one can use 20-25% H2SO4. Electrolysis method Preparing rust by electrolysis can be very cheap if you have suitable power source, a 9-12V transformer capable to give 5-20A will be enough in this experiment. Current should be rectified. This method is preferred if big amounts of rust is needed. Purity of product will depend on purity of used iron. Procedure Materials: Iron (plate or block) (Fe) Sodium chloride (NaCl) Power source (9-12V) Prepare a water solution of table salt (NaCl). As a rule of thumb I use 100 g NaCl for every litre of water. Concentration of salt depends on amount of current you want to run through solution. Ofcause current also depends on distance between electrodes and their size. It is usually more convenient to adjust current by changing concentration of NaCl. Find suitable electrodes. As anode (+) you can use any iron object, that you want to dissolve. Cathode (-) can be iron or copper. A bucket with fixed electrodes. As anode I used broken pliers. Note, I made a little "knot" on cathode near bottom, this will "rearrange" potentials a bit and make sure that anode will be eaten from bottom to top. Mount in place prepared electrodes in a plastic bucket with 10-20 cm spacing between. Connect (+) to anode (that suppose to dissolve), and (-) to cathode. Fill bucket with prepared salt solution, make sure it doesn't reach connections. Run 9V between electrodes, let the bucket stand outside. Following reactions takes place during electrolysis: Anode: Fe ==> Fe2+ + 2e- Cathode: 2H2O + 2e- ==> H2 + 2OH- Dissolved iron will directly form insoluble hydroxide, muddy in color: Fe2+ + 2OH- ==> Fe(OH)2 You have to make sure that power source is able to provide required current, do not rape it running at its full capacity. Maintain the level of water because some will be evaporated. Electrolysis time can vary, depending on current and mass of anode that have to be dissolved, it can take hours to days. To convert 1 kg iron you'll need 960Ah, which means, having current of 15A you will need 960/15=64 hours to dissolve 1 kg iron. When you think that there have dissolved enough of iron, disconnect power. By now, liquid should have alot of mud coloured precipitate, which is mostly Fe(OH)2. Filter the solid and dry it. On drying it will change colour to brown, you can find reactions in another rust preparation text above (Iron/HCl method), reactions are similar. Powder dried "cake" and heat it as in first method above. In my experiment I have converted 75 g iron in 20 h at about 3,8 A in average. Which gave 102 g dried rust (iron had some impurities). Theoretically, if electrolysis cell would be 100% efficient and used iron was pure, same amount of iron would be dissolved by 3,6 A in 20 h. This tells us that cell is pretty efficient (>95%). Some energy was lost in form of heat. Note, in both preparations above I've carried out oxidation of Fe(OH)2 in air. Many nonorganic chem books recommend to oxidise Fe2+ salt with H2O2 or HNO3(with second one, outside!), either prior or after the precipitation with NaOH. And only then proceed with drying. However, that is to prepare analytical grade iron oxide. Drying in air gives enough quality for most amateur experimenter.