Apologies in advance; feeling a bit stupid about this...

At the start of my furniture making program about a decade ago, I made a walnut console table that I finished with *pure* tung oil. To this day, the table top looks incredible. The walnut shimmers with almost a holographic/rainbow/spectral quality to it. Because I was new at the time (and stupid) I didn't write down my process. Now, ten years later, I am in the process of transitioning to full time furniture making and I desperately want this finish in my arsenal for future jobs.

Thing I remember: I used pure tung oil, sanded the top to ~1000 grit, applied the finish outdoors on a hot/sunny July day, my hand burned from the heat because the oil got so hot, applied the oil with either a T-shirt or white scotch brite, initially flooded the top before wiping it down, let it cure in the garage

Things I might have done (but don't quite remember): thinned the first coat with spirits, sanded between coats (using white scotch brite), done 3-5 coats

Every time I went back and attempted this finish it looked awful. Obviously the heat and UV components stand out. Recently I have been running experiments with full strength vs tung oil with mineral spirits, sample boards sanded between 1000-2000 grit, flooding it vs thin coats, wiping off vs hand rubbing to generate friction/heat, UV lamps at UVA (365nm LED at 100W) and UVC (254nm halogen at 25W)... and I still haven't landed on it yet. I even have a cutoff from the original table that I have been testing with. I put the original table under a microscope and it just looks either like the finish is permeated deep into the wood fibers or that it almost has layers to it. The other notable thing from the scope is that the grain is filled well (but not completely) with oil.

What's the critical component for getting tung oil to "shimmer"? If it involves the sun somehow, how can I recreate this in my garage shop in Michigan in January?

FWIW- I have been making furniture for a living for 27 years now. I have probably made over 700 pieces including sets of chairs etc. The amount of pieces that did not have stain on them or a custom color are less than 5. And that is from one client. Everything else has been stained or matched to some color.
The chatoyance look that you are after may be based on the angle of the grain on the original piece and not necessarily how your finishing it with oil now.

Interesting, first off I have to ask, if by " shimmer" your talking of chatoyancy... Or distinction of Image (DOI), If not then be more specific as to your meaning please. The beauty of wood mostly has to do with how figured the wood is. For example a plain flat sawn piece of walnut is pretty plain looking, where as a highly figured piece with various figures, such as burls, waves, Crotch, feathers, quilt, mottle, curl, etc., can be incredibly beautiful and eye catching, in comparison.
"DOI" Is what makes all wood appear as being more eye appealing, than ones who's surfaces are dull and muddy, looking, because of less refined sanding and prep and the use of pigments that have opacity, that hide the clarity and depth of the wood surfaces they are applied to.
Dyes help overcome this to a large degree, but are not light fast as natural earth pigments, that was and is why most high end furniture Mfg. Used a combination of both, to make a much more acceptable end product.
There are many finishes that give great to outstanding DOI, as to both depth, and figure enhancement, oils, for sure are good for this, but shellac, Nitrocellulose, Acrylics, polyester/epoxy, and Acrylic Urethanes, are among the best.! As to Tung Oil, there are two main ones on the market, monopoly me fixed and polymerized. The first has the benifits of being more fluid and easy to thin for better penetration, all on it's own, eventually it will polymerize,fully, but takes much longer than a pre Polymerized oil, and I find the Pre- Polymerized to give a header finish much quicker than the Non..!!
As to my method of using and applying, since 1969, I would sand the wood to 320 grit, if no dye or pigmented stains were used, or to the most 600 grit, silicone carbide.."wet/dry" paper if dye/pigment stains were being applied. I would then apply the prep polymer Tung thinned 4 parts high flash Naptha, to 1 part Tung, the (HF Naptha) gives better penetration. I would then cover or drape the object(s), with plastic 1-2 mil., so as to further the penetration of the oil, then check every few hrs. To see if it was all absorbed before putting the next same coat on. The second coat, I would use a non abrasive white scouring disc on my ROSander, and work the oil into the wood, and then leave overnight to dry, uncovered.
The next day I would use my 50/50 mix of Tung and this time, mineral spirits, apply heavily, wait till it was dulled, than again scour the surface till uniform in look, a satin type of uniform sheen. I would do 2-3 coats this way with drying a day or more, then start using the full strength P Tung and scouring method till I achieved the glossy look that I desired..!! or if the Customer preferred a less glossy more satin finish.
Your free to experiment with this, I will let you be the judge of the end product.
Chemmy

Pete, The original tabletop was made from a bunch of 3" strips of walnut in a butcher block pattern / array. They were leftovers from a variety of boards, a mix of sap and heartwood. But the walnut had been steamed and so the difference between sapwood and heartwood was mostly tonal; the sapwood was more of a light amber brown color. The slight variegation in color was done intentionally.

To your point though -- all of the boards in the tabletop demonstrate the same chatoyant shimmer despite coming from different timbers.

I do have a few old cutoffs to experiment with once I start to get close in the process. (I suffer from a sickness of hoarding shorts from old projects)

Yes, S.Wells is a good example, of P.Tung oil.!