Sugar Work
Every candy "stage" -- soft-ball, firm-ball, hard-crack -- is really just a checkpoint for how much water is left in the syrup. The thermometer isn't measuring an arbitrary number; it's measuring concentration.
The kitchen answer
A candy thermometer isn't reading "doneness" -- it's reading how much water has boiled out of the syrup. Less water left means a firmer, harder result once it cools, all the way from soft fudge to glassy hard candy, out of the exact same pot of sugar and water.
What's actually happening
Pure water boils at 100°C at sea level. Dissolve sugar into it and the boiling point rises -- a real, predictable effect called boiling point elevation: the more sugar relative to water, the higher the syrup has to get before it boils. As a sugar syrup cooks, water keeps evaporating, the sugar concentration keeps climbing, and the temperature climbs right along with it. That's the whole trick behind a candy thermometer: it's not reading "doneness," it's reading concentration.
What that concentration becomes when the syrup cools decides the texture. Around 85% sugar (soft-ball, ~112–116°C) there's still enough water for a soft, pliable set -- fudge, fondant. Push past 99% sugar (hard-crack, ~149–154°C) and there's almost no water left to plasticize it: the sugar sets glassy and snaps instead of bending.
What does that mean for your recipe
Same sugar, same pan, same heat -- fudge (soft-ball) and hard candy or a sugar-work showpiece (hard-crack) differ only in how far you let the water boil off before you stop. That also means the thermometer reading only means what it says at standard atmospheric pressure: at altitude, water boils at a lower temperature to begin with, so every stage shifts down too -- a "hard-crack" 152°C at sea level is not the same syrup as 152°C a mile up.
What can go wrong
- Grainy fudge instead of smooth: crystallization -- a stray sugar crystal (from an unwashed pan wall) or too much agitation while the syrup is hot enough to seed a chain reaction of crystal growth. Corn syrup, an acid, or just resisting the urge to stir all fight this. Full breakdown →
- Scorched, bitter batch: uneven heat -- sugar concentrated near a too-hot pan wall burns locally before the thermometer (reading the average) even registers it.
- Overshot the stage: carryover cooking -- a syrup pulled right at temperature keeps climbing for a moment from residual heat in the pan, especially in a heavy pot.
- Wrong stage despite a correct-looking thermometer: altitude or an uncalibrated thermometer -- both shift what temperature actually corresponds to a given concentration.
- Hard candy turning soft and sticky in storage: sugar is hygroscopic and pulls atmospheric moisture back in once unsealed, especially in humid weather. Full breakdown →
Try it yourself
One pot of sugar syrup, three checkpoints:
- Cook a plain sugar-water syrup on a thermometer.
- At 114°C (soft-ball), 138°C (hard-ball), and 152°C (hard-crack), drop a small spoonful into a cup of cold water.
- Compare texture (pliable vs. brittle), color, and smell at each stage.
Watch for: the same syrup moving from soft and pliable, to firm but bendable, to a glassy snap -- and the smell shifting from neutral to noticeably toasted as caramelization begins alongside the concentration change.
Go deeper
Heat and acid (or the enzyme invertase) hydrolyze some of the sucrose in a syrup into glucose and fructose -- a process called inversion, and the resulting mix is invert sugar. Because glucose and fructose don't pack into the same crystal lattice sucrose does, invert sugar physically interferes with crystal formation. That's the real reason a splash of corn syrup or lemon juice keeps fudge smooth: it's not a folk trick, it's deliberately seeding the syrup with molecules that get in sucrose's way.
More on the way
Next for this section: a full worked caramel recipe with the exact stage explained at each step.