Combine 4 cups of sugar and the milk or half-n-half in a large, heavy saucepan. (Rub the sides of the saucepan with butter, to help prevent graininess.) Stir and set aside.
Put the remaining 2 cups of sugar in a large iron skillet over medium heat. Stir constantly until the sugar starts to melt. At that time, place the sugar-milk mixture over low heat, stirring occasionally until the sugar dissolves.
At the same time, continue melting the sugar in the skillet, stirring, until all is melted and it is the color of light brown sugar. Melting sugar scorches VERY easily, so watch carefully. The entire process may take almost 30 minutes; at the end, you want one pan of light-brown melted sugar AND the milk-sugar mixture at a very light simmer.
The next step requires family teamwork. Pour the melted sugar into the simmering milk-sugar mixture in a stream "no bigger than a knitting needle". Stir constantly! This step may take five minutes, and works best if someone strong pours the melted sugar v-e-r-y s-l-o-w-l-y.
Continue cooking the combined mixture to the firm ball stage (246 degrees; higher at high altitudes), do not stir, other than to scrape the sides of the pan occasionally.
Remove from the heat at once. Stir in the baking soda--the candy will foam vigorously, so call the children to watch this step. Plop the butter into the foaming mixture, and let everything stand without stirring for 30 minutes.
Add vanilla and beat with a wooden spoon until the mixture stiffens and loses its gloss. This process may take 10 to 15 minutes, so beat in turns with your helper . Add the pecans, stir to mix, and turn the candy out into a buttered 13-by-9-inch rectangular pan.
Let the candy cool until barely warm; cut in smallish pieces--it's rich.
This candy is a wonderful keeper if each piece is wrapped in aluminum foil, and the wrapped pieces stored in an airtight container.
Aunt Bill's Brown Candy is traditionally sent by the women in our family to family members serving in the military. It's sturdy, keeps well and won't be damaged in transit--and brings an unforgettable taste of home to our family members in service.