There are several types of sand used in sand casting. Each has it's own advantages, disadvantages and use. On this page I'll try and provide some information on the different types of casting sand. There are a lot of different sands that are not mentioned here, the ones here are the more common ones used in the hobby foundry.
First green sand isn't green and no, I don't know why it's called green sand. In it's simplest form green sand contains sand, water, and clay. It requires less equipment do work with and is a good choice for the hobbyist. Green sand can be broken into 2 major types, natural and synthetic. Natural sand would be the type you would dig out of the ground if you're lucky enough to have a deposit near you. Properties of natural sand vary depending on where it was from. The clay content will be about 11 to 30%, the clay is usually kaolin. If you have sand near you there is nothing wrong with trying it, I haven't as there was no sand deposit near me to give it a shot.
Synthetic sand seems to be the more common way to go. The term synthetic sand is a little misleading, the sand isn't actually synthetic. What it means is you start with clean, graded sand in the grain size you want and then add the clay on your own. This allows you to more closely control the properties of the sand.
There are three basic types of clay that you can add to your sand;
Each of these clays has its own specific properties. The bentonites are more commonly used as they have more bonding power. This brings two advantages;
Western bentonite results in a greater dry/hot strength than the other two types. Southern bentonite has a higher green strength and greater permeability than the same quantity of Western, but only moderate dry/hot strength. For this reason Southern bentonite, or a mixture of the two, is a good choice for aluminum casting.
The sand you pick involves a few choices. If you are casting hotter metals such as copper alloys or cast iron you would want a coarser sand to allow the gasses to escape faster. I used 125 mesh sand that was sold for use to make brick mortar. It was cheap and available locally. Olivine sand is a good sand to form synthetic green sand from. It has some properties which lend it well to this purpose. It conducts heat better than most other sands which results in better chilling of the cast part. It also has no free silica, this means it can reduce the silicosis hazard. That said, the amount of casting I do as a hobbyist I'm not worried about silicosis caused by the small amount of dust I breathe in while working with green sand casting.
I've also seen a few synthetic sand recipes that add other ingredients to the mix such as wood flour. If I recall this was supposed to improve the surface finish with cast iron. I've not tried cast iron yet but was warned not to put anything in the sand which will biodegrade. After a few weeks the sand will still work but will smell very bad.
The recipe I used (based on Stewart Marshal's formula) resulted in what I consider to be very good results with aluminum. Listening to a lot of others there is apparently too much clay in this recipe. However I tried with 6-7 pounds of clay and could not get any green strength. This could be caused by a few thing; I used a very fine sand so there is a lot of surface area to cover, the sand I bought has a fairly rounded shape rather than angular, and I didn't mull the sand as I don't have a muller. Whatever the reason I couldn't get the green strength required with less clay. The water content is just a guess, I didn't actually measure it. I simple started out by adding a little water and working the sand through my fluffer a few times. I would then let the sand sit for several hours/overnight. I repeated this till it got to the point where it seemed right. When I pick up a handful of sand and squeeze it it will then hold its shape. I can then break it cleanly in half. The temper of the sand is something that you will have to learn by trial and experiment, at least I don't know any way to describe it to guarantee you'll get it right the first time. It is important to not use too much water as this will cause a lot of steam which will result in a poor casting, if there is a lot of extra water it is even possible that it could be dangerous as a result of the buildup of steam pressure blowing the molten metal out of the mold.
Green sand formula
Oil Tempered Sand
I do not have any of my oil tempered sand but, have used it when casting at a friend's place, so take what I say here with a grain of salt. Oil tempered sand has a special binder instead of one of the clays mentioned above. This binder reacts with oil rather than water like the previous clays. Since there is no water involved there is no steam generated when the metal is poured. This means that there is a reduced need for venting of the mold and the sand doesn't need to be as permeable. Because the sand can be less permeable you can use a finer mesh of sand which will result in a finer finish on the cast part. Also there is no water to evaporate so the sand will stay usable longer, even if it isn't in a sealed container.
That said, there are a few disadvantages with the oil tempered sands. The sand should be mulled when you are first making it. After the initial mulling you can get by with a fluffer/aerator until you need to add some more binder or oil at which time it should be mulled again. There are a few people who have built their own mullers and they seem to work well. It is possible for the oil vapour to ignite if you shake the mold out too early, this is not likely but something to be aware of. I've been told that it's not the best choice for cast iron work, to use green sand instead, although I have heard of people doing cast iron with oil tempered sand.
So if you have some way to mull the sand and are working with lower temp metals there is a lot to recommend the oil tempered sands. There are several brand names for the binder of oil tempered sand, Petrobond seems to be the most universally recognized. I won't give instructions for Petrobond as you should follow the instructions you receive with it when you buy it. There is also a homemade version, referred to as K-bond, which was developed at Kent State University. The following quote is from a message of Tom Cobett's on the hobbicast email list.
The Metal Casting program at KSU had been using Petrobond for about 20 years. One day we got a visit from the local environmental person who wondered if the building was burning down. (we had just finished pouring) When they found out that we were putting motor oil into sand and then causing it to burn by pouring liquid metal into it, well, let's just say that we were told to stop using Petrobond - IMMEDIATELY!
There are a number of different binders available for core sand. I'll only cover the two most commonly used by the hobbyist, baked core and sodium silicate. When I'm referring to baked cores I'm not referring to some of the new, higher tech types that involve a heat setting resin. I'm referring to what would have been used in the early days of foundry work. They are low tech which means they use technology which is available to the average hobbyist. The advantage to these cores are the low cost and easy availability.
Sodium silicate (waterglass) is a liquid that you mix with your core sand, for this you want plain clean sand. When you pack the sand in the core box the core needs to be hardened. There are a few way to do this, sodium silicate hardens in the presence of carbon dioxide. Depending on the size and complexity of the core you may be able to get it to harden by just leaving it exposed and the CO2 in the air will harden it, this could take a long time. Or you could gas the core box with CO2. There are a few ways to get CO2, you could get a tank and regulator from a welding supply shop. This would be very expensive if you are only doing a few cores. You could make some sort of adapter to make use of the small CO2 cartridges used for paint/pellet guns. Or, the cheapest, you could make use of chemistry. Combining vinegar and baking soda results in CO2, you just need to control where the gas goes. The advantage of the CO2 process is that you don't need access to an oven, and the extra equipment required is relatively simple and cheap. The cores made with this method will be as accurate as your corebox since you gas the core while still in the box and don't remove it till it's hardened.
Below is another quote from one of Tom Cobett's posts on the hobbicast email list.
When you ask for silicate, it is a bit like asking for vegetable oil. There are MANY types. Silicates are defined by the ratio of the parts of Silicon Dioxide to parts of Sodium Oxide. The most commonly available silicate has 3.22 parts of Silicon Dioxide for every one part of Sodium Oxide. Unfortunately, this stuff makes lousy foundry cores and molds. The ratio of the best silicates for foundry binders are from 2.40 to 2.60.
Baked core formula 1(Mark Fowler's formula)
Baked core formula 2 (tested successfully by Rupert Wenig)
Sodium silicate formula