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are filaments of slag
Slag is a partially vitreous by-product of smelting ore to separate the metal fraction from the unwanted fraction. It can usually be considered to be a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and metal atoms in the elemental form...
left in wrought iron
thumb|The [[Eiffel tower]] is constructed from [[puddle iron]], a form of wrought ironWrought iron is an iron alloy with a very low carbon...
after the production process. In their correct proportions their presence is beneficial, as they help to control the ductility
In materials science, ductility is a solid material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire. Malleability, a similar property, is a material's ability to deform under compressive stress; this is often characterized...
of the finished product, but when the proportion of slag is too high, or when the filaments run at right angles to the direction of tension, they can cause weakness.
Wrought iron is no longer made. The particles of slag present in the iron after preparation by puddling
Puddling was an Industrial Revolution means of making iron and steel. In the original puddling technique, molten iron in a reverberatory furnace was stirred with rods, which were consumed in the process...
were drawn into long fibres during the forging or rolling process. The proportion of slag was intended to be about 3%, but the process was difficult to control and examples with up to 10% slag were produced.
Stays made from puddled iron bar were used as a cheaper alternative to copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
for joining the inner and outer firebox plates of steam locomotives. The incorporated stringers gave flexibility akin to stranded wire rope and stays made of the material were therefore resistant to snapping in service. Wrought iron rivet
A rivet is a permanent mechanical fastener. Before being installed a rivet consists of a smooth cylindrical shaft with a head on one end. The end opposite the head is called the buck-tail. On installation the rivet is placed in a punched or pre-drilled hole, and the tail is upset, or bucked A rivet...
s made from iron bar typically contained stringer filaments running the length of the rivet, but filaments at right angles to the tension, particularly beneath the head, caused weakness.