Welding is a fundamental industrial fabrication process, the type of alloy used to make the weld is dependent upon the physico-chemical demands of the final structure, from aircraft to bridges to buildings to vehicles, each has its critical construction requirements. As discussed in a previous post, welding is a complex chemical process, sensitive to impurities and contaminants in the raw materials and the fabrication process itself to such an extent that it can affect the microcrystalline structure of the resulting alloy or produce a different bonding alloy altogether. 
Failure of a weld can have fatal consequences and could be indicative of a systematic error in the fabrication process, that is why it is important to evaluate every part of the welding process from development and pre-fabrication to post fabrication. Today we apply more science to our creations to get a better performance from our building materials and the construction process.
To evaluate the quality of metals and alloys used to construct our creations and the welds that bond them together we use increasingly more sophisticated techniques to detect macro and micro inconsistencies in our materials and processes, these techniques are attenuated dependent upon the metal or alloy used and the specific attribute being investigated by the selective use of chemicals and chemical blends. These products can reveal the general structure of a metal or alloy or, more specifically, areas of over-heating in the manufacturing or welding process, the different phases in a metal or alloy or the grain boundaries and a whole lot more. 
These products, used correctly, are designed to evaluate the macro and micro quality of metals & alloys and any weld used to bond them together; they are important teaching aids for welding instruction or self-instruction and development and they are indispensable diagnostic tools in building and infrastructure forensics. But most importantly, they are a cheap, simple and effective way of evaluating the fundamental characteristics of a metal and their alloys.