Why does my invert sugar always crystallize? I often use invert sugar as a humectant in confections.  The problem is that it always crystallizes after a few days.

My recipe is fairly simple.


1 kg refined sugar
200 ml filtered water
1 g cream of tartar


Bring to boil at 110C.  Let cool.

The syrup is definitely inverted, as the taste is distinct.  The invert sugar is also completely transparent which tells me there are no obvious undissolved sugar crystals.

Crystallization typically begins slowly at the point where I insert a spoon to portion out the syrup for recipes.

I store the syrup at room temperature in a glass syrup jar.  I have had no luck boiling the jar in water to liquify the syrup, as it appears saturated and it returns to the crystallized state.  Also I suspect the boiling temperature is above 100C, which the waterbath cannot exceed.
The crystalization is to be expected, that's what inverted sugar does. It is the same thing as with honey, which is chemically nothing but a contaminated inverted sugar. So, you are not doing anything wrong. In an inverted sugar at sucrose ratios, you will get sucrose crystalizing back out of the solution, just as you are observing now. 

If you really need it smooth, you can try changing the ratio by adding some fructose, as such a mixture is less prone to crystalizing. 

The one thing that puzzles me in your description is that you cannot get it back smooth by heating, this should in principle be possible. Maybe try adding a bit of water to the syrup itself during heating, so it can dissolve again.