Manufacturer and builder 11, 1869
From what has been explained (page 80) concerning chrome orange, it might be expected that chrome red would be a very basic chromate of lead. It may be obtained of various shades from a beautiful deep orange to a perfect vermilion red, with which, when it is properly prepared, it may almost compete in glow and depth of color. In this case it is sometimes called American vermilion. Unfortunately, however, the very deep varieties of this red color depend more on the peculiar molecular condition of the particles than on its chemical composition. The color is darker in proportion as the crystals of which the powder is composed are larger; and when paints of different shades of dark red are, by rubbing or grinding divided into smaller particles, the color becomes orange red. This is one simple test to distinguish this red from genuine vermilion, which does not change at all by this treatment. An adulteration of the last with chrome red is not to be feared, as it is impracticable. The sulphur in the vermilion forms with the lead in the chrome red a blackish compound which ruins the color, as all sulphur vapors in general spoil chrome red, as well as the chrome yellow made with lead, and in general all lead colors. Adulteration of chrome red itself has been found impossible, as by it the color in any case becomes inferior to the cheaper red lead. If the dark chrome red or American vermilion is to retain its beautiful deep red color, it must not be rubbed up with the oil or gum water, but simply mixed; the same is the case with certain greens, as Schweinfurth green, which, like Paris green, is an arsenite of copper.
There are four methods of preparing chrome red which we will separately explain, as each method pro-duces a peculiar product differing in shade from the others.
First way of making chrome red.
- If a soluble basic or neutral lead salt, or chloride of lead, or the oxychloride of lead, is mixed with a solution of yellow or red chromate of potash until the chromate of potash is in slight excess, there will be formed a chromate of lead which will be yellow or orange, according as the lead salt was neutral or basic and the chrome solution red or yellow. The light precipitate after being well washed is exposed to the action of a concentrated caustic solution; in this it will gradually become more red and contract to a heavy powder, and the snore so as it is treated in this way for a longer period.
As soon as this precipitate shows no further change, even by the addition of more caustic solution, the supernatant yellow fluid is removed and saved, as it contains chromate of potash, and therefore may be used for making a subsequent solution of this salt. The red precipitate is washed repeatedly, filtered and dried, by which treatment a mass is obtained which of itself falls into powder.
This red is usually slightly dark in color; if, how-ever, the solution and materials used were very pure, it will possess a brightness surpassing that of all the other chrome reds obtained by other methods.
It can, however, be made darker in a muffle-furnace; in this it is heated to a dark cherry red. It will become much darker, and nearly of the color of that made by means of, nitre, as described below. By repeatedly taking samples during this operation, the point at which the color does not intensify any further is easily observed. As soon as this is the case, it is withdrawn.
Second way of making chrome red.
- Woehler and Liebig have taught the art of making the darkest shades of red by heating chrome yellow with nitre. In place of treating the chrome yellow with an alkali, it is simply washed and dried. Nitrate of potassa is melted in a crucible heated to a low red heat and the dry powdered chrome yellow gradually added till little is left of the nitre, which is decomposed with effervescence. The temperature must not be too high. The crucible is then removed and placed aside, when the chrome red falls to the bottom of the mass of melted hot nitre. The remnant of this nitre is poured off, and after the crucible has been cooled, it is placed under a small stream of water, which is collected in another vessel. This stream of water is necessary, as if the solution stands long in the crucible the chrome red will be colored brown. At last the chrome red will become loose and freed from the adhering salt, when it must be well washed in a large vessel with water. The yellow fluid and the salt poured from the crucible contain much chromate of potash, and are therefore adapted to precipitate chrome yellow or orange from lead solution, to be treated as above if required to change them into chrome red.
The red thus obtained is very crystalline, and almost a perfect vermilion red. This is the method by which the most beautiful and desirable chrome reds are obtained.
Third way to make chrome red.
- Take a hot solution of nine parts neutral chromate of potash, to which is added some caustic potassa; or take three parts bichro-mate of potash, to which six parts of crystals of carbonate of soda dissolved in a sufficient amount of water are added, and mix either of these solutions with eight parts pure white lead finely ground in water, or even chloride, or oxythloride of lead. The lead salt will be slowly transformed into a fine chrome red, which in color is between the two kinds that we have just described, and also has a crystalline appearance. In accordance wtth the kind of white lead or chloride of lead, the liquid is able to transform more or less white into red; however, it is essential to have an excess of the caustic liquid, in order to prevent any unchanged white from becoming mixed with the red. This caustic liquid may, however, be utilized by pouring it over fresh white lead, preparatory to its treatment as above; or it may be used as if it were a simple solution of soda, and chromate of potash added, to snake the first solution mentioned. Time precipitate is, of course, washed and dried as above.
Fourth way of making chrome red.
- A very cheap and fine red, which however is inferior to the above, may be obtained by grinding pure litharge in water in a paint-mill to the highest degree of fineness, and boiling it with the gradual addition of chromate or bichromate of potash. Either of the last two substances must be added only so long as the color of the litharge gains in intensity. In using the simple chromate of potash, a slight excess does not matter much; but when using the bichromate, the least excess snakes the color at once orange, which, however, may then be corrected by adding a sufficient amount of litharge. All these particulars in the shades of color obtained are in perfect accordance with what was before ex-plained, in the article on the production of different shades of chrome yellow - page 52 of this volume.
Ei kommentteja :
Lähetä kommentti