Home Tests Show How Aluminum Gives Us Dyes and Paints

Popular Science, helmikuu 1937

by Raymond B. Wailes.

The electrical method of making aluminum amalgam by creating arcs between aluminum-foil electrodes and a drop of mercury. It is adcisable to wear tinted goggles, to protect the eyes against glare.

Paper impregnated with lead acetate reveals the presence of sulphur in ordinary laundry "bluing"

After dipping in mercuric chloride, a discarded aluminum tea ball sprouts tendrils of alumina

To perform a variety of entertaining experiments with aluminum and its compounds, an amateur chemist need not look far for materials. The kitchen yields the metal in the form of old pots and pans and their lids; spoons, tea balls, and salt and petter shakers. Powdered aluminum, sold for use in paint, provides another source of supply. Aluminum sulphate is familiar to scientific gardeners, for this compound of aluminum is commonly used to counteract excessive alkalinity of the soil. Alum from the household medicine chest - a compound chemically known as potassium aluminum sulphate - can be used directly in some experiments, and also will enable you to manufacture aluminum salts that you otherwise would have to purchase.

Dissolve some of the alum in water and add sodium hydroxide or ammonium hydroxide solution to it. Household lye or ammonia will serve the purpose. The jellylike precipitate that is formed consists of aluminum hydroxide. Was it by decantation - that is, let it settle, pour off the clear upper liquid, add freh water, and shake, repeating the process several times. Then strain out the precipitate. It must be filtered from the liquid with a cloth, as water would drain off from it too slowly through filter paper. Now you can prepare other compounds of aluminum at will, by dissolving the paste in various acids. For aluminum sulphtate, sulphuric acid is used; for aluminum chloride, hydrochloric acid; and so on. If you jeat aluminum hydroxide strongly you will get aluminum oxide, also known as alumina.

Many metals yield colorful sulphides when hydrogen sulphide gas bubbles through solutions of their salts. Aluminum sulphide, however, interacts with water and decomposes as fast as it forms, producing aluminum hydroxide. Most of the compounds of aluminum are white, like this one.

Ultramarine, familiar as a pigment in household laundry bluing and in blue paints, is an exception. Artists once ground up the rare mineral lapis lazuli to obtain it. Nowadays it is made synthetically. It contains aluminum in combination with silicon and other substances. You can prepare it on a small scale, using for ingredients about ten parts of kaolin (the white clay used in making porcelain), ten parts of sodium carbonate, and five or six parts of sulphur. Put the mixture in a porcelain crucible, cover it with a bit of charcoal, and set the lid of the crucible in place. Heat the contents with a Bunsen-buerner flame fr fifteen minutes. Then pour out the mixture into a flask and boil it with some potassium sulphide solution. This brings out the blue color. the kaolin used in this experiment, which supplies the aluminum, is known also as China clay, and may be obtained from chemical supply houses and from many drug stores. The potassium sulphide often goes by the name of liver of sulphur.

The "bluing" you have made contains sulphur in the sulphide form, as you can easily show by droppping some acid upon a fragment of it. You will instantly notice the foul odor of hydrogen sulphide gas, liberated from the sulphide by the acid (P.S.M., Jan. '36, p.54). If you moisten a piece of paper with lead acetate or lead nitrate solution, and hold the strip near the acid-treated bluing, the paper will turn brown or black, because of the formation of lead sulphide.

You can try out a peculiar method of waterproofing fabrics, in which aluminum plays a part, by mixing a solution of aluminum sulphtate with one of lead acetate. The heavy, white precipitate of lead sulphate that settles from the liquid is not concerned in the experiment. The clear liquid remaining contains aluminum acetate. When cloth is dipped in this olution, the aluminum compound soaks into the fibers. It turns into aluminum hydroxide by interaction with moisture, and impregnates the fabric. In drying, the cloth becomes so water-repellent that it can be wet only by the most strenuous efforts.

Artificial coloring for aluminum articles is a recent development, and you can duplicate this interesting process in your home laboratory. The secret lies in forming a coating of aluminum oxide upon the metal and then impregnating the exide, which acts as a sort of adhesive, with a dye or a chemically produced pigment.

Boil an aluminum object or a strip of the metal in a solution of sodium or potassium dichromate and sodium carbonate. This will produce the desired coating of oxide. Then wash it, and it will take on the chosen tint when it is immersed in a dye solution. The film of aluminum oxide "absorbs" or takes up the dye so that the metal itself appears colored. A yellow hue may be obtained without recourse to a dye, by immersing the oxide-coated aluminum first in a lead acetate solution and then, without washing, in a solution of sodium or potassium chromate. This precipitates a yellow chemical pigment, lead chromate, in the oxidized coating.

Another process, now employed commercially, forms the aluminum oxide electrically. The article to be colored is placed in a twenty-five percent solution of sulphuric acid and connected to the positive terminal of a source of direct current. If the tank containing the acid bath is of lead, it is connected to the negative terminal; otherwise, a strip of lead immersed in the acid serves as the negative electrode. Within certain limits of temperature and current strength, the coating of oxide is produced, and then is dyed or otherwise colored as before.

Making colors adhere to aluminum with the aid of its oxide recalls the common use of aluminum compounds as "mordants" to assist in dyeing cloth. In this case, aluminum hydroxide is the compound employed. To demonstrate its affinity for coloring matter, dissolve some Congo red dye (obtainable from dealers in chemical supplied) in about 300 cubic centimeters, or roughly ten fluid ounces, of water. Add aluminum hydroxide to the dye solution and boil the liquid for several minutes, swirling it about in the beaker or flask every minute or so. Now pour the liquid into a piece of filter paper folded in a funnel. the filtrate, or liquid that passes through the filter, will be clear, showing that the dye has been "adsorbed" by the aluminum hydroxide remaining on the filter paper. Almost any other dye solutions, such as colored inks that you may have about the house, will give the same result, and you will find it interesting to try them. A quantity of aluminum hydroxide sufficient for this experiment can be prepared by adding ammonium hydrozide to about ten grams (two teaspoonfuls) of alum dissolved in water. The precipitate should be washed by decantation, as explained in an earlier paragraph, before you use it.

To illustrate the use of aluminum compounds as "mordants" to aid in dyeing cloth, a piece of white fabric is dipped in solutions that precipitate aluminum hydroxide in its fibers

By comparing the effects of varous dyes on untreated cloth and the "waterproofed" cloth that you prepared in another experiment, you can observe the part that aluminum hydroxide plays as a mordant in actual dyeing. Dyes that have no effect on the plain cloth will effectively color the piece impregnated with aluminum hydroxide. Another way to prepare cloth for this dyeing test is to dip a piece of white fabric first in aluminum sulphate or alum solution, and then in dilute ammonum hydroxide, to precipitate the aluminum hydroxide in its fibers.

Powdered aluminum, sold for use with a varnish as aluminum paint, gives a dazzling wgite light when it is blown into a Bunsen-burner flame. The metallic powder liberates hydrogen when it is heated with sodium hydroxide solution, providing a convenient laboratory supply of the gas. By mixing finely divided aluminum with powdered magnetite or magnetic (ferric) oxide, the welding preparation known as thermite is obtained. Ignite it in a crucible, and the aluminum combines with the oxygen of the iron oxide and leaves the iron. So much heat is generated in the showy reaction, one of the most spectacular in chemistry, that sparks of molten iron fly from the white-hot mixture and soar through, the air like meteors. After the experiment, a button of solidified iron is found in the crucible.

Neither a match nor the flame of a Bunsen burner gives enough concentrated heat to touch off the thermite mixture. A small heap of a starting preparation must be placed on top of the chemicals and ignited. This may consist of equal parts by weight of potassium chlorate and sulphur. Another formula uses two parts of aluminum powder, two parts of potassium perchlorate, and one part of flowers of sulphur, also measured by weight. Do not use potassium chlorate in the latter preparation, as it would then be too sensitive to shock and might take fire or explode with slight friction.

Aluminum and mercury combine with each other to form an amalgam or alloy of curious behavior. Sandpaper a sheet of aluminum until it is clean and shiny, drop it into a bottle or flask, and shake it with several drops of mercury. You will not need much of the quicksilver; in fact, the small amount reclaimed from the bulb of a broken thermometer will be plenty. The agitation makes the mercury adhere to the cleaned surface of aluminum, or "amalgamate" with it. Now, if you remove the aluminum strip and expose it to the air, small white tufts of featherlike hairs will begin to grow upon it. You can actually see them getting longer as you watch. They consist of alumina, or aluminum oxide.

Immerse the amalgam-coated strip in water and it will decompose the liquid, liberating hydrogen gas. You can prove it is the amalgam that does this, for if you place a small drop of mercury and a plain strip of aluminum in water nothing will happen until they touch each other.

Electric current offers another way of making the aluminum-mercury amalgam. Roll up two pieces of aluminum foil, taken from candy bars or photographic film, and connect them with battery clips to a storage battery pr a group of dry cells. House current may be used instead, if a heating element of 600 or 660 watts is interposed in the circuit. Holding the pieces of foil by the insulated part of the wires attached to them, touch the aluminum strips simultaneously to opposite sides of a drop of mercury, and then pull them away. A brilliant arc is produced. It is a good precaution to wear a pair of tinted goggles to protect the eyes, and the fumes rising from the arc should not be breathed, as the mercury vapor is poisonous. One or two arcings are sufficient to produce the amalgam. Lay down the pieces of foil, watch them for a minute or two, and you will see the mosslike growth of white alumina appear.

The same result can be produced still more effectively by immersing a discarded aluminum tea ball or salt shaker for half a minute or so in a hot solution of mercuric chloride (bichloride of mercury), taking the special precautions that are due this poisonous chemical. Wash your hands carefully after using it, and make sure that nothing you have placed in the solution finds its way back to the kitchen by any mischance.

Remove the dipped object, rinse it with water, and hold it exposed to the air. it becomes warm as the white feathery fronds appear on it, and at times cannot be held in the hands. You can repeatedly wipe off the strange growth, wash the article in water, and dry it with a cloth - and still, after half a monite or so, the alumina tendrils will sprout again! As much as a teaspoonful of the peculiar white substance can be collected from the object.

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