This is a chemical process that puts a reflective silver layer on top of a mirror. It can be use for primary and secondary. This is the oldest method of putting a reflective layer on a glass substrate. For the best results you will need a bit of experience. The main disadvantage is that the silver layer will tarnish in a relatively short time.
High humidity is also to be avoided.
Depending on your local conditions a silvered mirror may last between 4 weeks and 1.5 years. Storing a H2S absorber along with the mirror can increase this life. One can also put a silver cloth over the mirror when it is not in use. The main advantages are that you don't need to ship your mirror and you can do it yourself. It is also cheaper than aluminizing, at least in the short run.