Ben:
You and Gabe are moving into the technical realm of metallurgy with the study of slag. There are three things that affect the content and appearance of slag:
1. The temperature of the furnace;
2. The type and quantity of flux used;
3. The type and constituency of ore employed in producing the pig iron.
In the 1908 work, The Blast Furnace and the Manufacture of Pig Iron, author Robert Forsythe states:
By the uninitiated, slag is too often looked upon as a highly undesirable, but quite often unavoidable excoriation from metallurgical processes. The proper mental attitude toward a slag is to consider it a reagent divinely appointed for the purification of metals. Since in any metallurgical process, all of the non-volatile constituents must appear in either the metal or the slag, it follows that whatever we would eliminate from the metal must be accommodated in the slag. This result can be accomplished only by giving to the slag such character that it will offer to the impurity a stronger attraction than is offered by the metal (p. 157).
To the practised eye, the appearance of the slag tells much concerning its composition and the temperature of formation. A slag that is high in earthy bases has a light gray to bluish granular fracture when cold. When high in lime, slags, “slake,” or crumble to powder soon after cooling. This tendency is retarded by the presence of magnesia. As the proportion of bases decreases, the slag shows vitreous tendency—at first on the outer edges only, but as the acid constituents increase, it may become glassy throughout. The presence of alumina opposes this tendency, and gives a more earthy appearance than silica alone. Siliceous slags can be drawn out into fine strings just before solidifying, while basic slags are very short. Hot acid slags, when run into the granulating pit, froth up into light fluffy heaps, while basic slags sink quietly to the bottom. Other things being equal, the slag from a cold furnace will be more vitreous than from a hot furnace. This is because it is more siliceous since less silicon has been reduced from the slag and gone into combination with the iron.
A siliceous slag is more likely to carry an appreciable quantity of oxide of iron than a basic slag. This is because unsatisfied SiO2 will seize upon any unreduced iron that reaches the fusion zone more readily than when it is saturated with bases. For this reason slags from a cold furnace generally have a dark color and high specific gravity, particularly when improperly prepared material is projected into the hearth. Calcareous slags sometimes give a dark color when a furnace is in trouble without losing their earthy appearance or containing an undue proportion of rion. The dark color appears to be simply a stain, probably caused by finely divided carbon (p. 166).
Writing in his 1852 book, A Treatise on Metallurgy, Frederick Overman notes concerning meadow or bog iron:
Bog-ore [is] brown hematite [or] hydrated oxide of iron. …This is a very abundant iron ore, and a source of cheap metal; it forms the bulk of ore in this country. Hematite is essentially a hydrated peroxide, with definite quantities of water, which vary from 9 to 13 per cent. In its purest form it contains from 50 to 62 per cent, of metal. The varieties of this ore are very numerous; it occurs in all shades of color, from black to a faint yellow. …Bog-ore often contains from ½ to ¾ per cent. of phosphorus.
…Phosphoric acid is frequently found in iron ores; quite as well in those which are primitive as in those of the coal formations and younger ores. Phosphoric acid in contact with coal is converted into phosphorus; and as iron has strong affinities for phosphorus, we always find it in the metal if it has been in the ore or fuel—particularly in gray metal. …Phosphorus, unlike sulphur, causes iron to be very fluid even in small quantities and at low heats. Owing to this property, phosphorus is less vexatious when present in iron than sulphur. Iron with phosphorus is white, close, and compact; assumes a high polish, and is less attacked by oxygen than other alloys. It is extremely brittle, so that the least force will break it when cooled below 32°. Phosphorus will drive sulphur from iron, when the latter is present; still, they may be both in crude iron at the same time. Sulphur is removed before phosphorus can be evaporated. Iron which contains phosphorus melts easily, works well in refining, is easily welded, and is in fact very managable (pp. 471, 477).
In 1932, John Lord Bacon wrote about forges and its slag in his volume, Forging:
Wrought iron is made in what is called the puddling furnace. Into this furnace is charged pig iron, which is heated to a molten state and kept boiling. A temperature in excess of 2150° Fahrenheit is required to melt the pig iron. The atmosphere in this furnace is oxidizing, that is, the oxygen combines at the high temperature with the carbon and impurities in the pig iron (and even with some of the iron itself); these are burned or oxidized out, leaving an iron with little or no carbon, silicon, and manganese in it. To assist this oxidation, men operating the puddling furnace continually stir up the molten iron with long poles of green wood or bars of iron. When the carbon, silicon, and manganese have been removed, the iron is no longer in a liquid state but is in a solid pasty condition, sometimes referred to as a weldable condition. This solid condition of the iron results from the raised melting point due to a purer iron being present after burning out the impurities. The melting point of pig iron, such as remains, melts around 2800° Fahrenheit.
In this pasty condition the iron is removed in small balls dripping with slag, formed from the oxides of the different impurities, and is placed into a squeezer. This removes some of the slag and welds together the iron, by squeezing the iron closer together, something on the idea of squeezing water out of sponge. After this squeezing operation, the iron is passed through rollers to be formed into muck bars of wrought iron. This material is used as a base for the manufacture of wrought iron and steel, but the muck bars cannot be used commercially in this form.
To further refine the muck bars and make them into a good grade of wrought iron, the bars are piled into a furnace, heated to a welding heat, and then rolled or hammered into commercial bars. The more working the iron receives the better quality the wrought iron becomes. The working at a welding heat further welds together the iron and expels the slag. What slag is left in the iron is elongated into long thin fibers (pp. 4-5).
Obviously, not everything quoted above applies to Pineland furnaces and forges. Nevertheless, the basic metallurgical chemistry does not change. I have many other works in my library that deal with the production and processing of iron and steel. Let me know if you and Gabe would like to avail yourselves of these sources.
Best regards,
Jerseyman
