Hello from Denmark

So you want to cast aluminum. Forget about plaster of paris.  OK for low meting point pewter etc.

there are two casting methods to look at Sand casing AKA green sand casting. You make a pattern traditionally made from wood . you build a flask made in two parts to hold sand bound with clay . you form a cavity by packing sand around the pattern . patting powder keeps the sand from the upper and lower parts of the mold apart. remove the pattern cut paths for the aluminum to flow assemble the mold and pour in the aluminum . This is the easy economical way   .The pattern is used over and over again so is the sand.

second method based on lost wax casting. print a pattern from PLA or better yet wax if you can find it.  add sprue or print it on the pattern place in cylinder (flask )pour investment around pattern  heat  cure investment much like firing ceramic. Use centrifugal caster to inject  molten metal into hot flask. Let cool break away flask.

Note this is brief overview not a how to or step by step instruction. If you choose to do home casting do you homework . plan your work use proper safety equipment. 

Spilled molten metal can damage many things it touches, catch things on fire, burn flesh etc A hidden drop of water can cause an explosion and literally send molten metal towards your face. 

I am not saying this to scare  you but be aware home casting can be rewarding to the skilled trained and informed but can seriously injure the unaware and unprepared.
Tin

Honestly I would not do any melting, or pouring of molten metals of any kind in or near my house, garage. I would do them in my backyard preferably in an area covered in sand or dirt. No vegetation or other flammables like leaves. When you smelt the metal avoid breathing the smoke as it is usually toxic. The danger of spillage is great and keep in mind that even lead which is a lower melting point will still be around 800C which is more than enough to catch most materials on fire. Aluminum will be around 1200 and well you get the picture as you use harder metals.

And melting point of lead Is about 328C not sure where you are getting your numbers.

Bronze melting point About 950c . 1200 c is very near the melting point of cast Iron.

And yes a bed of sand is the best place to do a pour. Concrete and asphalt to be avoided.
centrifical casting done typically inside a washtub.
Tin