Flux is a broad term used for ingredients used to change the characteristics of the melt, these can be physical and chemical changes, there are also quite a few different things which are used to make up a flux recipe, some of these are many times normal ingredients, then there are also some that are added to change the chemistry of the melt, for each type of ore there is no one recipe, because not all ore is the same, each rock has its own chemistry and the flux used is normally determined by that ores specific chemistry, many of the flux ingredients can be the same so many times a general recipe is chosen and then chemicals or materials added to adjust the flux recipe to get the desired results with that particular ore, these are usually determined by test melts of the ore, and adjusting the flux chemistry according to these test melts to get a recipe of flux that works the best to extract the and reduce desired metals into metallic form and to oxidize the undesired metals and other materials up into the slag glass.
Stock fluxes can be mixed up and then adjusted as needed.
Stock flux may consist of a basic flux ingredients like:
Borax, sodium carbonate (soda ash, or washing soda), quartz sand or crushed ground glass.
Then they may add as needed:
A tiny bit of fluorspar (or fluorite ore), to help viscosity.
A metal collector, lead oxide, copper silver...
Flour, corn meal, or other carbon source as a reducer.
Sodium nitrate or other oxidizer.
Or other ingrediants.
I have seen many different recipes for a house flux many are similar.
But really to learn this you will need to do a lot of study as smelting is a chemistry skill in its own right.
There is no magic flux recipe for all ore, learning what each of the ingredients you have in your flux will do to the melt chemically, will help you to determine how to adjust your flux recipe to best fit your ore, and to get the most of the valuable metal that you can from that ore.
As with other chemistry it is an art and skill that will require Quite a bit of study, and gaining experience to get good at it.
It is not as simple as mixing up A + B + C into a flux and melt it with ore and get gold, you will need to do a lot of research and learn quite a bit before you will be able to do this skill.
I have studied it for a while and experimented with it a bit as a hobby, well enough to learn how complicated it can actually get, I did collect some house recipes along the way, but found also the ingredients would need adjusted varied so widely even in the house flux depending on the ore I was trying that, I started thinking that even the house flux should be determined by the type of ore.
Find some good books if you wish to learn, there have been several links and downloads to some great books on this subject here on the forum, buying C.W Ammens (Action mining sells this book) is a book that can help learn some about this and other mining techniques,