Dice Balance

Most handmade dice are balanced; however, the majority of dice - including factory-made sets - are not perfectly, 100% balanced. The general process of dice made by humans makes it impossible to guarantee perfect balance, but regardless, they are suitable for casual play. The only truly balanced dice are dice used by casinos. Even those dice are only considered balanced for approximately 4 to 8 hours before they are switched out. Sometimes, casinos will switch out earlier than that if dice become too "hot" (ex: a person is rolling extremely well so the casino wants to make sure the dice haven't been tampered with). Below are some things to consider when thinking about and discussing balance of dice. 

Shape

The shape of the dice (ex: miniscule changes from polishing) isn't going to have a noticeable effect on the balance of the dice. Notably, mass-produced dice tend to be a little bit more oblong (I'm sure you've seen the pictures/videos on this), but are still within good reasonable tolerances. Beyond this, because of how handmade dice are polished and made, they're typically more accurate.

The reason Casino dice have to be so perfect is sheer quantity of rolls, and the fact that one entity (the casino) can monetarily benefit from uneven dice. Casino dice are measured with digital calipers, and have to have be true per face to 0.0005 of an inch.

A number that comes up 1% more often will affect one out of every hundred rolls. It might be several sessions before an RPG player has this crop up, while a casino will have it crop up a few dozen times per day. And again, the casinos make money on this. We're just having fun, and in pretty much any situation, we can just roll with whatever number comes up in whatever game we're playing. Oh no, we missed the bandit! We'll get them next turn.

Rolling

How you roll your dice, the friction, material, and give of the surface you roll onto, the oil and creases in your hands, the specific strength of gravity and elevation where you play, microseismic activity, temperature, etc. All of these things affect your rolls and therefore affects the balance of the dice.

Furthermore, the way the dice themselves interact with the surface they're rolling on is a factor. Sharp edged dice on softened dice trays roll radically different than round edge dice do. Casinos try to mitigate that with bumpers, specific tabletops, etc. Smooth surfaces tend to make dice slide. "Dropping" dice will yield a different result than letting them go out of the side of your hand or off the end of your fingers. Rolling towers or other mechanical rolling devices (ex: dice crossbow or dice catapult or even a dice cup) will also yield different results. 

Inclusions

The density of pretty much any inclusion (3d printed resin, flowers, glitter, micas, inks, store-bought plastic or rubber items) is going to be on par with the density of the resin used for the dice. These inclusions are so small that they don't even register on a scale! As such, it won't have a noticeable effect on the way the die rolls.

Small internal bubbles also won't affect the balance. Opaque, mass-produced dice can have imperceptible bubbles that can't be seen until you saw them open, and won't be noticed even with a salt bath.

Speaking of salt baths.... The salt bath test cannot ever prove that a die is balanced. It can only prove that it is unbalanced. It doesn't take into account the surface of the die and how it interacts with things, or how it's rolled, etc. You can have a die that has rounded corners on one side and hard corners on another and it'll still come up "balanced" even though it'll roll funny.

In Conclusion

On a long enough timeline, over a thousand-plus rolls, every single die that has ever been made, from "perfect" casino dice to Chessex to artisan, will show a bias. This is due to wear and tear, micro abrasions, throw variations, physics, etc. Every. Single. One. 

These dice are being used for a game. If worrying about the balance of the dice is going to ruin your entertainment of the game, then handmade dice are not for you, dear buyer. However, you should also perhaps consider why you have never questioned the balance of your factory made dice.

As makers, we are in no way ever forcing anyone to buy or use our dice.