How to get a glossy glaze, and why yours goes dull
Shine is a physical property, not a recipe. Four things destroy it and one move creates it.
The dark, mirror-shiny coating on a good plate of sticky chicken or a soy-braised pork belly is not an accident of the recipe, and it is not something you can get by simply reducing the sauce further. Reducing further is, in fact, one of the reliable ways to lose it.
Gloss is a physical property, and it comes from a small number of controllable things.
What makes a glaze shine
A surface looks glossy when it is smooth enough to reflect light in one direction rather than scattering it. Three things get you there.
Sugar concentration. Enough dissolved sugar makes a syrup that sets to a smooth film as it cools. Not enough and it stays watery and dull.
Fat, properly dispersed. Fat fills in the microscopic roughness. This is why a sauce finished with a little oil or butter goes from matte to shiny in seconds.
Starch, if used, at the right moment. Gelatinised starch gives a translucent, clinging body. Mishandled, it gives the opposite.
Everything below is about not wrecking one of those three.
The four ways it goes dull
Boiling after the starch goes in
This is the common one. A cornflour slurry thickens a sauce the moment it hits the heat, and keeping it at a rolling boil afterwards breaks the starch back down. The sauce thins, goes cloudy, and loses its shine.
Add the slurry at the end, let it come to a gentle bubble for fifteen to thirty seconds, then stop. Not four minutes. Not a hard boil.
Too much starch
Over-thickened sauce reads as pasty and matte rather than glossy, and it coats the mouth unpleasantly. A glaze should be thin enough to run slowly off a spoon while still hot. It will thicken as it cools, and judging the consistency while it is at full heat is how people end up with gravy.
Overcooking the sugar
Sugar that goes past caramel into the start of burning turns dark, bitter and dull. In a kicap manis braise this happens quietly, because the sauce is already dark and you cannot see it coming. Go by smell — sweet and toasty is right, sharp and slightly acrid is past it.
Adding the fat too early
Fat added at the start of a long reduction mostly separates out and floats. Fat swirled in at the end, off the heat, disperses through the sauce and is what actually produces the shine.
The finishing move
Take the pan off the heat. Add a small amount of oil or butter — a teaspoon or two for a dish serving four — and swirl the pan rather than stirring hard.
The sauce will visibly tighten and go glossy. This takes about ten seconds and is the single biggest difference between a home version and a restaurant one.
Then plate it immediately. A glaze left sitting in the pan will dull as it cools and separates.
Reduction or starch
Both work and they give different results, which is worth knowing before you choose.
Reduction concentrates everything: flavour, salt, sugar, colour. The gloss is deeper and the flavour is more intense. The cost is time, a smaller volume of sauce, and the risk of over-salting, since everything you seasoned earlier is now concentrated.
Starch gives you volume and shine without further concentration. It is faster and more forgiving of seasoning, which is why a busy dai chow kitchen uses it constantly. The flavour is less deep.
Most good sticky dishes use a little of both: reduce until the flavour is right, then a small slurry to bring the body and shine up to where you want them.
Why commercial versions look different
Packaged sauces and many restaurant glazes use glucose syrup or maltose rather than plain sugar, and they shine in a way that is genuinely hard to reproduce with granulated sugar alone.
Maltose in particular gives that thick, lacquered look on char siu and roast meats. It is sold in tubs at Chinese grocers, it is extremely sticky to work with, and warming the tub in hot water before you try to spoon any out will save you a certain amount of swearing.
If a dish you are copying looks glossier than anything you can achieve, maltose is often the missing variable rather than technique.
A note on salt
Reduction concentrates salt faster than it concentrates anything else, and a glaze is by definition reduced.
Season light at the start and correct at the end. A glaze that has come out too salty cannot be fixed by adding water, because you will lose the gloss you just spent twenty minutes building. The only real fix is more of the unsalted components, which means more sugar, more liquid, and starting the reduction again.
The general principle here is the one in fixing a dish that tastes wrong: it is much easier to add than to remove, and reduction removes nothing.