I've recently purchased some wood to make some sayas. I was originally going to use thin basswood to make a sandwich style, but am now I'm contemplating trying one or two by chiseling out the space for the blade. What size chisels would I need? Also, what type of mallet would people recommend for this type of work? TIA for any help!
Basswood is soft and pretty forgiving. It's a great wood to try your hand at carving with. But what size chisel is best will kind of depend on what type of knife you're carving for. Ideally, you want to try and strike a balance between a width that is wide enough to cut relatively quickly, but small enough to give you control when carving the tapers/angles. I like to aim for a chisel that's no wider than 1/3 the blade height but that's an arbitrary reference.
I find 5/8th (16mm) to be a good universal option, in general. If doing a tall gyuto or a nakiri, you can do a lot of the work with a slightly larger blade, and if doing a yanagiba or something shorter in height, 3/8th or 1/2in will give you more control. I also like to keep either a 1/4 inch or a skew tip chisel for cleaning up edges with more precision, or working near the tip.
If you are trying to hone your skills, arguably more important than chisel size is chisel type. There are many profiles and styles, not unlike our kitchen knives. Bench chisels (the sort of home-depot, universal do all chisel) that most people think of and are quick to slap silly with a mallet aren't usually great for precision carving work. The bevels can easily cause you to dig into the wood if not careful about reading the grain direction, and the shorter length of their neck doesn't give you much leverage (or knuckle clearance) if you're going to try and use them free hand. with a mallet, you lack some control and have a greater risk of gouging or chip-out. Mortise chisels are usually pretty stocky and while good for both hand cutting or a mallet, depending on the cut depth and direction, they are the chisel equivalent of a chopping tool. From the western tool box, the chisel to go grab is going to be a paring chisel. Longer, with a thinner neck that may flex a little, these are ideal for doing slower, controlled hand cuts. Once you've outlined your shape, you can shave away material quickly and cleanly with a sharp paring chisel. It's lighter style won't be as prone to dig into the wood, and they do a great job paring off thin shavings cross-grain - ideal for the angles involved in getting a saya cavity that matches the blade tapers and bevels. *( If you don't have a paring chisel, look at socket chisels. They usually are longer than generic bench chisels, though still a multi-use design that can be used with a mallet or freehand.)
All this said - basswood is forgiving, so it's worth jumping in and just giving it a go ... Because it's a soft wood, as long as your chisels are sharp, you should be able to get a good result regardless of chisel type or sizes.
As for mallets - I like wooden mallets for chisels...but apart from outlining my cut (which I prefer to use knife for), I don't use a mallet for much work on sayas unless it's really dense hardwoods. I use one hand to push the chisel and the other to put pressure on the chisel neck or just behind blade...this allows maximum control.
if your curious to browse specialty woodworking tools, look at highland hardware, traditional woodworker or lee valley.