NO! IT MUST BE COLD! I was a cook in the navy and I have the secret to the no miss corn starch thickening trick...trust me - this works the best! Thickening any kind of sauce or gravy is a tricky thing and you can screw it up bad if not careful. Take a small bowl and add 4 Tablespoons of cold water. Take 4 Tablespoons of cornstarch and add to the cold water and mix quickly with a fork. When the mixture is blended add to your sauce you wish to thicken. Mix in with the gravy/sauce briskly and wait to see if it needs another "dose" of corn starch. The secret is equal amounts of cold water to equal amounts of cornstarch mixed thoroughly. If it needs to be thickened more, repeat the process I detailed earlier. A sauce or gravy is thick enough when you dip a spoon into mixture and it stays medium coated after you lift it out.