Paint Technology and Tests Part 6

(Greatly magnified)]

[Ill.u.s.tration: White Lead (Quick Process)]

On account of its alkaline nature, this pigment acts upon the saponifiable oil in which it is ground, forming lead soaps which accelerate chalking of white lead--the greatest evil attending its use.

Solubility in carbonic acid of the atmosphere and decay in the presence of sodium chloride may be active causes of the rapid chalking of this pigment at the seash.o.r.e. Checking in some climates appears to proceed rapidly on white lead paints, in a deep hexagonal form, leaving a series of rough crests and cracks. This checking is secondary to the chalking which takes place.

[Ill.u.s.tration: Corrosion cylinders used for making Quick Process White Lead]

[Ill.u.s.tration: Lead Melting Pots]

=White Lead (Quick Process).= By acting on atomized metallic lead, contained within large revolving wooden cylinders, with dilute acetic acid and carbon dioxide, the quick-process white lead is produced. Its value is equal to the Dutch-process white lead, and it is considered by some as possessing greater spreading value.

[Ill.u.s.tration: Sheet iron box luted at bottom with water. Atomized lead, blown into box with steam, falls to bottom and becomes hydrated (Mild Process)]

[Ill.u.s.tration: _Photographs courtesy of Stowe Neal_

View of agitation tanks for making Mild Process Lead]

[Ill.u.s.tration: Steam Jected Pans for Drying White Lead]

=White Lead (Mild Process).= The Mild Process of manufacturing white lead consists of first melting the pig lead and converting it into the finest kind of lead powder, then mixing thoroughly with air and water.

The lead takes up water and oxygen and forms a basic hydroxide of lead.

Carbon dioxide gas is next pumped slowly through the cylinders which contain the basic hydroxide of lead. The result is basic carbonate of lead--the dry white lead of commerce. The process is called "Mild"

because it is the mildest process possible for the manufacture of white lead. It is the only method in practical operation which does not require the use of acids, alkalis or other chemicals, every trace of which should be removed from the finished product by expensive purifying processes. The failure of such washing and purifying means a product of inferior quality, which necessarily reduces the durability of any paint in which it is used.

=Basic Sulphate-White Lead (Sublimed White Lead).= By the action of the oxygen of the air on the fume produced by the roasting and subsequent volatilization of galena, this fine, white, amorphous pigment is made.

On a.n.a.lysis, its composition shows approximately 75% of lead sulphate, 20% of lead oxide, and 5% of zinc oxide. It has a specific gravity of 6.2. Possessed of extreme stability, it finds wide use as a const.i.tuent of paints and as a base for tinting colors. The photomicrograph of this pigment shows its extremely fine, amorphous nature with complete absence of crystals. In fineness it closely approaches zinc oxide. On account of its non-poisonous properties it is replacing corroded lead in many places. Unified paints containing sublimed white lead are of great value, showing upon long exposure very little decay.

[Ill.u.s.tration: View of Furnace for Making Sublimed White Lead]

[Ill.u.s.tration: View of Goosenecks Used for Collecting Sublimed White Lead Fume]

[Ill.u.s.tration: Bag Room Where Sublimed White Lead is Deposited

_Photographs courtesy of Picher Lead Co._]

[Ill.u.s.tration: Sublimed White Lead]

[Ill.u.s.tration: View of largest Zinc Oxide Works in America, at Hazards, Pa.]

=Sublimed Blue Lead.= Sublimed blue lead is made by burning coa.r.s.ely broken lumps of galena, admixed with bituminous coal, in a special form of furnace. The fumes which are volatilized from this mixture are very complex in their chemical make-up, and in color are white, blue, and black. After being drawn through the cooling pipes by the suction of huge fans, whereby the fumes are cooled, the pigment is deposited in bags. This pigment is bluish black in color, and has been highly recommended for use on iron and steel. Its composition runs approximately as follows:

Lead sulphate 50% Lead oxide 35% Lead sulphide 5% Lead sulphite 5% Zinc oxide 2% Carbon 3%

[Ill.u.s.tration: View of Zinc Oxide Furnaces]

[Ill.u.s.tration: _Photographs courtesy Geo. B. Heckel and N. J. Zinc Co._

View of Zinc Oxide Fume Pipes with electrically driven Suction Fans]

The color of the pigment is largely due to the carbon and the lead sulphide. Its specific gravity is 6.4, and it grinds in 10% of oil to a stiff paste, 100 lbs. of which may be thinned with about 26 lbs. of oil to working consistency. Paint manufacturers use it in mixture with iron oxide and other pigments for the production of paints for metal surfaces. Wood and others have found it of great value for this purpose.

It has a tendency to chalk, but this may be overcome by admixture with other pigments such as zinc oxide and iron oxide. Lane has found it to be very durable when admixed with lampblack.

[Ill.u.s.tration: View of Bag Room receiving Zinc Oxide]

=Zinc Oxide.= This extremely white and fine pigment is prepared by the roasting and sublimation of franklinite, zincite, and other zinc-bearing ores largely found in New Jersey. Its purity approaches in most instances 99.5 or more. It has a specific gravity of 5.2. On account of its stability, whiteness, and opacity, it is invaluable as a pigment when a const.i.tuent in a combination formula. Its extreme hardness renders it less resistant to temperature changes, when used alone. Under the microscope the fineness and structure of the particles are clearly evident. The French-process zinc oxide produced in America by the sublimation and oxidation of spelter is the purest made, and superior to imported grades which often contain ultramarine blue as a whitening agent.

[Ill.u.s.tration: Zinc Oxide]

[Ill.u.s.tration: Zinc Lead White]

[Ill.u.s.tration: Zinc Lead. By transmitted light

(_The Pigment shows black_)]

[Ill.u.s.tration: Lithopone]

[Ill.u.s.tration: Magnesium Silicate (Asbestine)]

=Zinc Lead White.= This extremely fine pigment, consisting of about equal parts of zinc oxide and lead sulphate, results from the reduction, volatilization and subsequent oxidation of sulphur-bearing lead and zinc ores. It has a specific gravity of 4.4. Its slightly yellowish tint bars it from being used alone very extensively, but when mixed with white lead, zinc oxide and inert pigments, or used as a base for colored paints, it is of considerable value. The magnification of the particles shows the peculiar way in which the pigment agglomerates, and the characteristics of a fine, uniform pigment.

[Ill.u.s.tration: Asbestine Mine at Easton, Pa.]

[Ill.u.s.tration: American Barytes. Transmitted light

(_The Pigment shows black_)]

[Ill.u.s.tration: German Barytes. Mag. 250 Diam.

(_The Pigment shows white_)]

=Lithopone.= Lithopone, probably the whitest of pigments, results from the double decomposition of zinc sulphate and barium sulphide, thereby forming a molecular combination of zinc sulphide and barium sulphate.

The peculiar property which it possesses, of darkening under the actinic rays of the sun, makes it essential that it be combined with other, more stable pigments to prolong its life when exposed to weather. Lithopone contains approximately 70% barium sulphate, 25 to 28% zinc sulphide, and as high as 5% of zinc oxide. Its specific gravity is about 4.25. It is excellently suited for interior use in the manufacture of enamels and wall finishes. When properly mixed with other pigments, such as zinc oxide and calcium carbonate, fair results are obtained as a pigment for outside work. Lead pigments are never used with lithopone, as lead sulphide results, giving a black appearance. Its characteristic flocculent, non-crystalline nature is plainly evident when examined under the microscope.

[Ill.u.s.tration: By Polarized Light

By Transmitted Light

Barium Sulphate (Barytes)]

=Magnesium Silicate (Asbestine and Talcose).= This pigment comes in two forms: as asbestine and as talcose (talc, etc.). The former is very fibrous in nature and is a very stable pigment to use in the manufacture of paint, on account of its inert nature and tendency to hold up heavier pigments, and prevent settling. It also has the property of strengthening a paint coat in which it is used. The talcose variety is very tabular in form. Both varieties are transparent in oil, and very inert. They have a gravity of about 2.7 and grind in about 32% of oil.

[Ill.u.s.tration: Barium Carbonate. Mag. 250 Diam.

(_The Pigment shows white_)]

[Ill.u.s.tration: Barium Sulphate (Blanc Fixe)]