Solder is an alloy of various materials. The most common ones for leading and copper foil work are tin, lead, copper and silver. The most important is tin. There are, of course, some solders that do not have tin in their composition.
The most common alloy for us is tin and lead. Various proportions produce different melting (liquidus) and solidification (solidus) points. This graph shows the effect of changing the amount of tin in a tin/lead solder.
This shows that 61.9% tin and 38.1% lead produces an eutectic solder (although others report a 63/37 alloy as eutectic). That is, a solder which has both its liquidus and solidus temperatures the same. This kind of solder solidifies very quickly after its melting. If we put a lot more heat into this kind of solder, it takes time to become solid. During that cooling, the solder bead can become disturbed and become either crystalline or marked. The objective should be to move quickly enough to melt the solder, but not to dwell, as that adds heat.
For the other common combinations [insert ref to previous blog entry] there is a temperature range where the solder is pasty. It is neither fully liquid (needed to get a good bead) nor yet solid. It is in this range that various problems can arise.
Failing to get the solder to the liquidus state will result in what is called a cold joint. The solder is crystalline at the visible level. It has visible cracks and will not adhere to the copper foil or lead properly. If disturbed while the temperature is in the pasty range while cooling from the liquidus state, you will also get a crystalline structure to the solder, resulting in an insecure joint.
The graph also shows the melting points of lead (327.5C) and tin (232C). The wonder of an alloy is that by combining these two metals, the solidus points are greatly changed. This graph shows is that tin is not fully solid until 13C, while lead is solid immediately below its liquidus point, but by combining them a solidus temperature of 183C is achieved.