Into a large saucepan over medium-high heat, whisk together the water, corn syrup, granulated sugar, and cream of tartar.
Bring to a boil, stirring continuously until mixture reaches 300 degrees F (hard crack stage) on a candy thermometer.
Working quickly, pour sugar mixture onto a parchment-lined, rimmed metal baking pan. Allow to cool completely.
When the glass is completely cool, use the edges of the parchment paper to lift it out of the pan.
Place on a smooth surface and cover with a second piece of parchment paper.