Part 55

The glazing or enamel mills are shown in Fig. 10. These mills consist of a strong iron frame securely bolted to a stone foundation. In the sketch shown the framing carries 2 mills, but 3 or 4 can be arranged for. A common arrangement for small factories consists of 2 large mills, and 1 smaller mill, driven from the same shaft. One of the mills is used for foundation or gray mixings, the second for white, and the smallest one for colored mixings. In these mills it is essential that the construction is such as to prevent any iron fitting coming into contact with the mixing, for, as has already been explained, the iron will cause discoloration. The ground plate is composed of quartz and is immovable. It is surrounded by a wooden casing—as shown at _a_—and bound together by iron hoops. The millstones are heavy, rectangular blocks of quartz, called “French burr stone,” and into the center the spindle, _b_, is led. The powdered material mixed with about three times its bulk of water is poured into the vats, _a_, and the grinding stones are then set in motion. When a condition ready for enameling has been reached the mixture is run off through the valves, _c_. Each mill can be thrown out of gear when required, by means of a clutch box, without interfering with the working of the others. The grinding stones wear rapidly and require to be refaced from time to time. To avoid stoppage of the work, therefore, it is advisable to always have a spare set in readiness to replace those removed for refacing. The composition of the stones should not be neglected, for, in many cases, faults in the enamel have been traced to the wearing away of stones containing earthy or metallic matter. {302}

«Enamel Mixing.»—All constituents of which an enamel glaze is composed must be intimately mixed together. This can only be done by reducing each to a fine powder and thoroughly stirring them up together. This part of the work is often carried out in a very superficial manner, one material showing much larger lumps than another. Under circumstances such as these it is absurd to imagine that in fusion equal distribution will take place. What really happens is that some parts of the mass are insufficiently supplied with certain properties while others have too much. A mixture of this class can produce only unsatisfactory results in every respect, for the variations referred to will produce variations in the completeness of fusion in the viscous character of the mass, and in the color.

The mixing can be done by thoroughly stirring the various ingredients together, and a much better and cheaper system is mixing in rotating barrels or churns. These are mounted on axles which rest in bearings, one axle being long enough to carry a pulley. From the driving shaft a belt is led to the cask, which then rotates at a speed of from 40 to 60 revolutions per minute, and in about a quarter of an hour the operation is complete. The cask should not exceed the 5-gallon size, and should at no time be more than two-thirds full. Two casks of this kind give better results than one twice the size. The materials are shot into the cask in their correct proportions through a large bung hole, which is then closed over by a close-fitting lid.

«Mixings.»—For gray or fundamental coatings:

I.—Almost any kind of glass 49 per cent Oxide of lead 47 per cent Fused borax 4 per cent

II.—Glass (any kind) 61 per cent Red lead 22 per cent Borax 16 per cent Niter 1 per cent

III.—Quartz 67.5 per cent Borax 29.5 per cent Soda (enameling) 3 per cent

The above is specially adapted for iron pipes.

IV.—Frit of silica powder 60 per cent Borax 33 per cent White lead 7 per cent

Fused and then ground with—

Three-tenths weight of silica frit. Clay, three-tenths weight of silica frit. Magnesia, one-sixth weight of white lead.

V.—Silica 65 per cent Borax 14 per cent Oxide of lead 4 per cent Clay 15 per cent Magnesia 2 per cent

No. V gives a fair average of several mixings which are in use, but it can be varied slightly to suit different conditions of work.

«Defects in the Gray or Ground Coating.»—Chipping is the most disastrous. This may be prevented by the addition of some bitter salt, say from 3 to 4 per cent of the weight of the frit.

The addition of magnesia when it has been omitted from the frit may also act as a preventive, but it should only be added in very small quantities, not exceeding 2.5 per cent, otherwise the temperature required for fusion will be very great.

«Coating and Fusion.»—Difficulties of either may generally be done away with by reducing the magnesia used in the frit to a minimum.

A soft surface is always the outcome of a mixing which can be fused at a low temperature. It is due to too much lead or an insufficiency of clay or silica powder.

A hard surface is due to the quantity of lead in the mixing being too small. Increase the quantity and introduce potash, say about 2.5 per cent.

The gray or fundamental mixing should be kept together in a condition only just sufficiently liquid to allow of being poured out. When required to be applied to the plate, the water necessary to lower it to the consistency of thick cream can then be added gradually, energetic stirring of the mass taking place simultaneously in order to obtain uniform distribution.

The time required for fusion may vary from 15 minutes to 25 minutes, but should never exceed the latter. If it does, it shows that the mixing is too viscous, and the remedy would be the addition and thorough intermixture of calcined borax or boracic acid. Should this fail, then remelting or a new frit is necessary.

A highly glazed surface on leaving the muffle shows that the composition is too fluid and requires the addition of clay, glass, silica powder or other substance to increase the viscosity.

As has been already explained, the glaze is much more important than the fundamental coating. Discoloration or slight flaws which could be tolerated in the latter would be fatal to the former. {303}

In glazes, oxide of lead need not be used. It should never be used in a coating for vessels which are to contain acids or be used as cooking utensils. It may be used in sign-tablet production.

For pipes the following glaze gives good results:

I.—Feldspar 33 per cent Borax 22.5 per cent Quartz 16.5 per cent Oxide of tin 15 per cent Soda 8 per cent Fluorspar 3.75 per cent Saltpeter 2.25 per cent

For sign tablets the following gives fair results, although some of the succeeding ones are in more general use:

II.—Cullet 20 per cent Powdered flint 15 per cent Lead 52 per cent Soda 4.5 per cent Arsenic 4.5 per cent Niter 4 per cent

III.—Frit of silica powder 30 per cent Oxide of tin 18 per cent Borax 17 per cent Soda 8.6 per cent Niter 7.5 per cent White lead 5.5 per cent Carbonate of ammonia 5.5 per cent Magnesia 4 per cent Silica powder 4 per cent

The following are useful for culinary utensils, as they do not contain lead:

IV.—Frit of silica powder 26 per cent Oxide of tin 21 per cent Borax 20 per cent Soda 10.25 per cent Niter 7 per cent Carbonate of ammonia 5 per cent Magnesia 3.25 per cent

This should be ground up with the following:

Silica powder 4.25 per cent Oxide of tin 2.25 per cent Soda 0.5 per cent Magnesia 0.5 per cent

V.—Feldspar 41 per cent Borax 35 per cent Oxide of tin 17 per cent Niter 7 per cent

VI.—Borax 30 per cent Feldspar 22 per cent Silicate powder 17.5 per cent Oxide of tin 15 per cent Soda 13.5 per cent Niter 2 per cent

Borax will assist fusion. Quartz mixings require more soda than feldspar mixings.

VII.—Borax 28 per cent Oxide of tin 19.5 per cent Cullet (powdered white glass) 18 per cent Silica powder 17.5 per cent Niter 9.5 per cent Magnesia 5 per cent Clay 2.5 per cent

VIII.—Borax 26.75 per cent Cullet 19 per cent Silica powder 18.5 per cent Oxide of tin 19 per cent Niter 9.25 per cent Magnesia 4.5 per cent Soda 3 per cent

To No. VII must be added—while being ground—the following percentages of the weight of the frit:

Silica powder 18 per cent Borax 9 per cent Magnesia 5.25 per cent Boracic acid 1.5 per cent

To No. VIII should be similarly added the following percentages of the frit:

Silica powder 1.75 per cent Magnesia 1.75 per cent Soda 1 per cent

This mixing is one which is used in the production of some of the best types of hollow ware for culinary purposes. The glaze should be kept in tubs mixed with water until used, and it should be carefully protected from dust.

«Defects in the Glaze or White.»—A bad white may be due to its being insufficiently opaque. More oxide of tin is required. Cracks may be prevented by the addition of carbonate of ammonia. Insufficient luster can be avoided by adding to the quantity of soda and reducing the borax. If the gray shows through the white it proves that the temperature of fusion is too high or the viscosity of the mixing is too great. If the coating is not uniformly spread it may be due to the glaze being too thin; add magnesia. If the glaze separates from the gray add some bitter salt. Viscosity will be increased by reducing the quantity of borax. Immunity against chemical reaction is procured by increasing the quantity of borax. An improved luster will be obtained by adding native carbonate of soda. The greater the quantity of silicic acid the greater must be the temperature for fusion. To reduce the temperature add borax. Clay will increase the difficulty {304} of fusion. Oxide of lead will make a frit more easily fusible. A purer white can be obtained by adding a small quantity of smalt.

«Water.»—The character of the water used in the mixing of enamels is too frequently taken for granted, for unsuitable water may render a mixing almost entirely useless. Clean water, and with little or no sulphur present, is essential. For very fine enamels it is advisable to use carefully filtered water which has shown, after analysis, that it is free from any matter which is injurious to any of the enamel constituents.

«How to Tell the Character of Enamel.»—In the case of sign tablets the characteristics looked to are appearance and the adherence of the coatings to the iron. For the latter the tests are simple. The plate if slightly bent should not crack the coating. An enamel plate placed in boiling water for some time and then plunged into very cold water should not show any cracks, however small, even after repeated treatment of this kind.

Culinary utensils, and those to hold chemicals, should not only look well, but should be capable of resisting the action of acids. Lead should never enter into the composition of enamels of this class, as they then become easily acted upon, and in the case of chipping present a menace to health. The presence of lead is easily detected. Destroy the outside coating of the enamel at some spot by the application of strong nitric acid. Wash the part and apply a drop of ammonium sulphide. If lead is present, the part will become almost black, but remains unchanged in color if it is absent.

Another simple test is to switch up an egg in a vessel and allow it to stand for about 24 hours. When poured out and rinsed with water a dark stain will remain if lead is present in the enamel. To test the power of chemical resistance is equally simple. Boil diluted vinegar in the vessel for several minutes, and if a sediment is formed and the luster and smoothness of the glaze destroyed or partially destroyed, it follows that it is incapable of resisting the attacks of acids for any length of time. There are several other tests adopted, but those given present little difficulty in carrying out, and give reliable results.

«Wasters and Seconds: Repairing Old Articles.»—In all enameling there must be certain articles turned out which are defective, but the percentage should never be very great. The causes which most frequently tend to the production of wasters are new mixings and a temperature of fusion which is either too high or too low. There are two ways of disposing of defective articles, viz.: (1) Chipping off the bad spots, patching them up and selling them as “seconds”; (2) throwing the articles into the waste heap. The best firms adopt the latter course, because the recoating and firing of defective parts practically means a repetition of the whole process, thus adding greatly to the cost, while the selling price is reduced. Overheating in fusion is generally shown by blisters or by the enamel being too thin in various places. Chipping may be also due to this cause, the excessive heat having practically fused the fundamental coating.

At this stage the defects may be remedied by breaking off the faulty parts, patching them up, and then recoating the whole. With sign tablets there is no objection to doing so, but with hollow ware the fact remains that the article is faulty, no matter how carefully defects may be hidden. As white is the most general coating used, and shows up the defects more than the colored coatings, the greatest care is necessary at every stage of the manufacture. While glowing on the article, it should appear uniformly yellow, but on cooling it should revert to a pure white shade. On examining different makes of white coated articles, it will be found that some are more opaque than others. The former are less durable than the latter, because they contain a large percentage of oxide of tin, which reduces the elasticity. To ensure hardness the mixing must be very liquid, and this cannot be arrived at when a large quantity of oxide of tin is introduced.

Old utensils which have become broken or chipped can be repaired, although, except in the case of large articles, this is rarely done. The operations necessary are: (1) The defective parts chipped off; (2) submitted to a red heat for a few moments; (3) coated with gray on the exposed iron; (4) fused; (5) coated with the glaze on the gray; (6) fused.

«To Repair Enameled Signs.»—

Copal 5 parts Damar 5 parts Venice turpentine 4 parts

Powder the rosins, mix with the turpentine and add enough alcohol to form a thick liquid. To this add finely powdered zinc white in sufficient quantity to yield a plastic mass. Coloring {305} matter may, of course, be added if desired.

The mass after application is polished when it has become sufficiently hard.

«Enamel for Copper Cooking Vessels.»—White fluorspar is ground to a fine powder and strongly calcined with an equal volume of unburnt gypsum, at a light glowing heat, stirring diligently. Grind the mixture to a paste with water, paint the vessel with it, using a brush, or pour in the paste like a glaze and dry the same. Increase the heat gradually and bring the vessels with the glass substance quickly into strong heat, under a suitable covering or a mantle of burnt clay. The substance soon forms a white opaque enamel, which adheres firmly to the copper. It can stand pretty hard knocks without cracking, is adapted for cooking purposes and not attacked by acid matters. If the glassy substance is desired to cling well and firmly to the copper, a sudden and severe heat must be observed.

«To Pickle Black Iron-Plate Scrap Before Enameling.»—The black iron-plate scraps are first dipped clean in a mixture of about 1 part of sulphuric acid and 20 to 22 parts of water heated to 30° to 40° C. (86° to 104° F.), and sharp quartz sand is then used for scouring. They are then plunged for a few seconds in boiling water, taken out, and allowed to dry. Rinsing with cold water and allowing to dry thus may cause rust. The grains of quartz cut grooves in the fibers of the iron; this helps the grounding to adhere well. With many kinds of plate it is advisable to anneal after pickling, shutting off the air; by this means the plates will be thoroughly clean and free from oxidation. Much practice is required.—_The Engineer._

«ENAMELED IRON RECIPES.»

The first thing is to produce a flux to fuse at a moderate heat, which, by flowing upon the plate, forms a uniform surface for the white or colored enamels to work upon.

Flux for Enameled Iron.—

White lead 10 parts Ball clay 1 part Flint glass 10 parts Whiting 1 part

The plates may then be coated with any of the following mixtures, which may either be spread on as a powder with a little gum, as in the case of the flux, or the colors may be mixed with oil and the plates dipped therein when coated; the plate requires heating sufficiently to run the enamels bright.

Soft Enamels for Iron, White.—

Flint glass 16 parts Oxide of tin 1 1⁠/⁠2 parts Niter 1 1⁠/⁠2 parts Red lead 4 parts Flint or china clay 1 part

Black.—

Red oxide of iron 1 1⁠/⁠4 parts Carbonate of cobalt 1 1⁠/⁠4 parts Red lead 6 parts Borax 2 parts Lynn sand 2 parts

Yellow Coral.—

Chromate of lead 1 part Red lead 2 3⁠/⁠4 parts Flint 1 part Borax 1⁠/⁠4 part

Canary.—

Oxide of uranium 1 part Red lead 4 1⁠/⁠2 parts Flint 1 1⁠/⁠2 parts Flint glass 1 part

Turquoise.—

Red lead 40 parts Flint glass 12 parts Borax 16 parts Flint 12 parts Enamel white 14 parts Oxide of copper 7 parts Oxide of cobalt 1⁠/⁠4 part

Red Brown.—

Calcined sulphate of iron 1 part Flux No. 8 (see page 307) 3 parts

Mazarine Blue.—

Oxide of cobalt 10 parts Paris white 9 parts Sulphate barytes 1 part

Fire the above at an intense heat and for use take

Above stain 1 part Flux No. 8 (see page 307) 3 parts

Sky Blue.—

Flint glass 30 parts White lead 10 parts Pearlash 2 parts Common salt 2 parts Oxide of cobalt 4 parts Enamel, white 4 parts

Chrome Green.—

Borax 10 parts Oxide of chrome 4 1⁠/⁠2 parts White lead 9 parts Flint glass 9 parts Oxide of cobalt 2 parts Oxide of tin 1 part {306}

Coral Red.—

Bichromate potash 1 part Red lead 4 1⁠/⁠2 parts Sugar of lead 1 1⁠/⁠2 parts Flint 1 1⁠/⁠2 parts Flint glass 1 part

Enamel White.—Soft:

Red lead 80 parts Opal glass 50 parts Flint 50 parts Borax 24 parts Arsenic 8 parts Niter 6 parts

Enamel White.—

Red lead 10 parts Flint 6 parts Boracic acid 4 parts Niter 1 part Soda crystals 1 part

Where the enameled work is intended to be exposed to the weather do not use flux No. 8, but substitute the following:

White lead 1 part Ground flint glass 1 part

All the enamels should, after being mixed, be melted in crucibles, poured out when in liquid, and powdered or ground for use.

«FUSIBLE ENAMEL COLORS.»

The following colors are fusible by heat, and are all suitable for the decoration of china and glass. In the following collection of recipes certain terms are employed which may not be quite understood by persons who are not connected with either the glass or porcelain industries, such as “glost fire” and “run down,” and in such cases reference must be made to the following definitions:

“Run down.” Sufficient heat to melt into liquid.

“Glost fire.” Ordinary glaze heat.

“Grind only.” No calcination required.

“Hard fire.” Highest heat attainable.

“Frit.” The ingredients partly composing a glaze, which require calcination.

“Stone.” Always best Cornwall stone.

“Paris white.” Superior quality of whiting.

“Parts.” Always so many parts _by weight_, unless otherwise stated.

“D. L. Zinc.” Particular brand not essential. Any good quality oxide of zinc will do.

Ruby and Maroon.—Preparation of silver:

Nitric acid 1 ounce Water 1 ounce

Dissolve the silver till saturated, then put a plate of copper in the solution to precipitate the silver in a metallic state. Wash well with water to remove the acetate of copper.

Flux for Above.—Six dwts. white lead to 1 ounce prepared silver.

Tin Solution.—Put the acid (aqua regia) in a bottle, add tin in small quantities until it becomes a dark-red color; let it stand about 4 days before use. When the acid becomes saturated it will turn red at the bottom of the bottle, then shake it up and add more tin; let it stand and it will become clear.

Aqua Regia.—

Nitric acid 2 parts Muriatic acid 1 part

Dissolve grain gold in the aqua regia so as to make a saturated solution. Take a basin and fill it 3 parts full of water; drop the solution of gold into it till it becomes an amber color. Into this solution of gold gradually drop the solution of tin, until the precipitate is complete. Wash the precipitate until the water becomes tasteless, then dry slowly and flux as follows:

Flux No. I.—

Borax 3 parts Red lead 3 parts Flint 2 parts

Run down.

Rose Mixture.—

Purple of Cassius 1 ounce Flux No. 1 6 ounces Prepared silver 3 dwts. Flint glass 2 ounces

Grind.

Purple Mixture.—

Purple of Cassius 1 ounce Flux No. 8 (see page 307) 2 1⁠/⁠2 ounces Flint glass 2 ounces

Grind.

Ruby.—

Purple mixture 2 1⁠/⁠2 parts Rose mixture 1 1⁠/⁠2 parts

Grind.

Maroon.—

Rose mixture 1 part Purple mixture 2 parts

Grind. {307}

Black—Extra quality.—

Red oxide of iron 12 parts Carbonate of cobalt 12 parts Oxide of cobalt 1 part Black flux A (see next formula) 80 parts

Glost fire.

Black Flux A.—

Red lead 3 parts Calcined borax 1⁠/⁠2 part Lynn sand 1 part

Run down.

Black No. 2.—

Oxide of copper 1 part Carbonate of cobalt 1⁠/⁠2 part Flux No. 8 (see next column) 4 parts

Grind only.

Enamel White.—

Arsenic 2 1⁠/⁠2 parts Niter 1 1⁠/⁠2 parts Borax 4 parts Flint 16 parts Glass 16 parts Red lead 32 parts

Glost fire.