it aint difficult , you just need to be very careful and remember all parts of it that you can repeat it exactly the same next time .
or you can make it a lot easier with some modern equipment as a pyrometer to know when you got the right temperature .
but if you weight the copper and thin and have the right temperature then it aint any more problems with the casting .
the problem is the mold making .
its many steps in that process .
please remember that when Im talking casting I always talk casting by the lost wax method .
thats approximately the same method as was used 4- 500 years ago .
but ok today you use some modern materials instead of what was used then .
start googling at backyard foundry and lost wax casting , then you have a week or so to read that
maybe it sounds difficult but it aint , it just take a hell of a lot of time to do it to get an perfect result , but its very fun and most of the work you can do on your living room table .
ok thats if you live alone of course
I do
but its a lot of shortcuts you could take if you only will produce one single item .
I would guess that you could produce one martar like this one but in golf ball size for 50 us dollars
but it will involve you in a hell of a lot of work also .
you can also invest 20 - 30 000 us dollars in equipment and have an very low quality as a result if you dont know what you do .
so my advise to you is to start googling and learn as much as possible in this topics . but forget anything called sand casting if you want an good result .