You can thicken your tomato juice with a roux, which is just a blend of fat and flour in a 1-to-1 ratio. About two tablespoons of roux will thicken a cup of liquid. For the fat, you can use oil or butter, depending on your taste preference. Olive oil works well for a pizza sauce. To make the roux, warm the oil until it's hot in a pan. Then add your flour, stirring constantly. You want to cook the flour and oil until it begins to color and the raw flavor of the flour cooks out. Don't cook it over high heat, as your roux might burn. When the mixture turns into a bubbling paste, add your spices and slowly whisk in the tomato juice. Continue stirring until the pizza sauce reaches the desired consistency.

Tomato paste is a thickening agent that is somewhat bitter. If you use it to thicken your tomato juice, heat the tomato juice and paste in your saucepan until you're happy with the texture. Add spices to your personal taste, and use a little extra sugar to offset the paste's acidity. You can also add crushed and/or stewed tomatoes for more flavor and a chunkier consistency. Just drain off the excess liquid from the crushed or stewed tomatoes before adding them to your sauce. If you want to add the tomatoes for flavor, but want a smooth sauce, put the sauce through a sieve after cooking.

A reduction is a simple way to thicken any sauce. This method cooks off the amount of water in the juice by applying heat. Start by putting the juice in a sauce pan and bring it to a rolling boil. Turn down the heat and let the sauce simmer. As it simmers, you’ll notice the amount of liquid decreasing. Stir periodically and continue simmering until you’re happy with the consistency. It takes at least one hour for 12 ounces of juice to begin thickening. At this juncture, add spices to your personal taste. Continue cooking to the desired thickness. This should make enough sauce for a large pizza, depending on how heavily you apply it. Alternatively, you can microwave your juice on high using a microwave-safe container. Reducing this way takes between 20 and 30 minutes to obtain 1 cup of sauce.

Flour, cornstarch and arrowroot are three potential thickening agents for your tomato juice. All three of these work best as a slurry, which you make by blending equal amounts of the thickening agent with cold water. Once mixed, slowly stir your slurry into your boiling, already seasoned, tomato juice. Be conservative when it comes to the amount of slurry you add. You can always add more if desired. Keep in mind that cornstarch will typically work better than flour, which may separate or lump. Arrowroot is more expensive than cornstarch.