Choose 9in (23cm) pans that are at least 2 inches (5.1cm) deep. Spray the inside with baking spray to prevent the marble cakes from sticking. To make a large, single-layer cake, use a 9in 13in (23cm 33cm) cake pan. Open a 15.25-ounce (432 g) box of yellow or white cake mix and put it in a large mixing bowl. Ignore the instructions on the box and instead add 1/2 cup (100 g) of granulated sugar and 1/4 teaspoon (1.5 g) of salt. Add 4 eggs and these liquids: 2 teaspoons (9.9ml) of vanilla extract 13 cup (79ml) of canola oil or vegetable oil 4 tablespoons (56 g) of melted butter 14 cup (59ml) of whole milk 1 cup (240ml) of water Turn a hand or stand mixer to low and beat until the eggs are combined. Then turn the mixer up to medium speed and beat the batter for 2 minutes. Stop the mixer and scrape down the sides of the bowl with a spatula once or twice. Reduce the mixer speed to low and gradually beat in 2/3 cup (83 g) of all-purpose flour or 3/4 cup (75 g) of cake flour. Beat just until the flour is mixed into the wet batter. Put 3 tablespoons (22 g) of unsweetened cocoa powder into a separate mixing bowl and add 2 tablespoons (24 g) of sour cream. Stir or whisk the cocoa until it's incorporated. Then stir in 1 cup (340 g) of the batter from the other mixing bowl. This will make the chocolate batter for the marble cake. Divide the plain white or yellow cake batter between the prepared cake pans or pour the plain batter into the large cake pan. Then dollop large spoonfuls of the chocolate batter over the plain batter in the pan. Leave about 2 inches (5.1cm) of space between the blobs of chocolate batter. Insert the skewer so it almost reaches the bottom of the pan and then drag it in a figure-8 pattern so it moves through several of the chocolate blobs. Then insert the skewer into another part of the pan and make another figure-8. Do this 1 to 2 more times so each chocolate blob is marbled. It's important to not over-swirl or the batters will mix and you won't be able to see the marbled effect. Put the cake pans in the preheated oven and bake them until they pull away from the sides of the pan. To test if they're done, insert a toothpick into the center and pull it out. If it comes away clean or with just a crumb, the marble cake is finished. If batter sticks on the toothpick, bake the cake for 3 to 5 more minutes and check it again. Turn off the oven and transfer the cakes to a wire rack to cool for 10 minutes. Carefully flip the cakes out of the pans and leave them to cool completely before you frost them with 3 cups (700 g) of your favorite chocolate buttercream, ganache, or icing. Cover the leftover cake and store it at room temperature for up to 2 or 3 days. You can also refrigerate it for up to 5 days.
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One-sentence summary -- Preheat the oven to 350F (177C) and grease 2 round cake pans. Put the mix, sugar, salt, eggs, vanilla, fats, milk, and water in a bowl. Beat the ingredients in the bowl for 2 minutes. Stir in the flour until it's incorporated. Mix the cocoa, sour cream, and 1 cup (340 g) of the batter in a another bowl. Spread the plain batter and spoon the chocolate batter on top. Use a skewer to swirl the batters. Bake the marble cake for 30 minutes. Cool the cake completely before you frost and serve it.

Rust bluing is the process of coating the metal in acid. Most commonly, nitric acid or hydrochloric acid is used. This allows the part to rust uniformly. Then, you scrub the rust off and repeat the process until the desired color is achieved. You can do this in a large container or vat that is tolerant to acid. Fume bluing is similar to rust bluing, but it creates a more uniform rusting. The part is placed in an airtight chamber and either nitric acid or hydrochloric acid (gas) is pumped into the chamber. The gas causes a uniform bluing. Any airtight container will work, as long as it cannot be corroded by the acid. Niter bluing is the hottest form of bluing. In this case, you take a mixture of potassium nitrate salt and sodium hydroxide salt and melt them (700F (371C). The part will achieve a nice bluing when dipped into the solution for. There are many types of bluing professionals. If you are not up to trying it yourself, you can reach out to a bluer to learn their specialty. Ask what kinds of bluing they do and what pieces they are willing to treat (e.g. large or small).
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One-sentence summary -- Rust blue antique parts. Fume blue for an even finish. Try niter bluing. Contact a bluing expert.

Place the dutch oven on your largest burner. Put the thermal ballast in the bottom of the pot and cover it with both lids. Turn the burner on high heat. Allow the heat to rise within the oven for 5 minutes. After five minutes, reduce the heat to just above medium. You want to flour all parts of the inside of the pan. It is helpful to rub oil or lard on the inside for the flour to stick to, then pour the flour in, turn and shake the pan until it is completely covered on the inside. The dough should fit well in the pan. The dough will rise more during baking, so it should not exceed the top of the pan. Another option to covering the pan with flour is using oatmeal. Pour oil in the pan covering the bottom and sides, then pour finely ground oatmeal in the pan. Twisting your wrist, be sure to turn and shake the pan until all the sides are also covered in oatmeal. Using an oven mitt, remove the lids of the dutch oven and set them aside on a heat-proof surface. Carefully place your loaf pan in the center of the dutch oven, avoiding the hot sides. Make sure there is enough room on all sides of the pan for heat to surround it. Using oven mitts, place both of the lids back on top of the dutch oven. Bake the bread for about 30 minutes. Check it after 20 minutes to see if it seems to be gaining a crust. The top of the bread will not brown, but it should no longer be soft and doughy once it is cooked. Remove the lids and the pan from the dutch oven using oven mitts. Carefully shake the bread out of the pan. It should fall out easily because you floured your pan. The bottom of the bread will be significantly more cooked than the top. If you don't have a cooling rack, you can cool the bread on another heat-proof surface like a plate.
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One-sentence summary --
Preheat the dutch oven. Place the dough in your floured baking pan. Place the pan inside your dutch oven. Bake the bread. Place the bread on a cooling rack.