Historically speaking, there were basically two camps of colored finish being used for the Lehigh-area "red" finishes. I should note that some were deliberately variations on this tone as well, a few being known that are a very rich amber/orange and some an amber/yellow. Tom covered the french polish above, which addresses the spirit varnish side of things. Some of these were very intensely colored by super-saturating the varnish with colored resin; this was done through careful heating. Watch out. I believe that the two most common tinting agents were probably the dragon's blood resin mentioned above as well as alizarin, although alizarin has proven to fade much more rapidly than dragon's blood resin. Some of these old finishes, however, will absolutely not dissolve with alcohol or anything short of very strong application of acetone or paint stripper. These appear to be oil based finishes as they are very tough. These are based on very finely ground iron oxide pigments - yes, I said the dirty word: pigments - mixed into an oil varnish. It is possible, not through grinding but through chemical action, to obtain various shades of iron oxide that are so finely "ground" (although there is no real grinding involved) as to be translucent. These will not muddy your grain right up as a traditional pigmented stain or finish will. For an example of such chemical action, one need only look at the sediment obtained through the process of making aqua fortis. This can be used in various ways to obtain colors from a yellow/gold up through a strong rusty red. I think you can purchase some of these today from a better quality art supply, however they will most likely be ground into a oil that is not particularly suitable for stock finishing (in regard to dry time - purified and slow drying versions of linseed oil or walnut oil, most likely).
