This is the video I was talking about. It's part 1 of a 3-4 part series.
In that video, he has 12 test pieces of purpleheart. He bakes 4 @ 250°F, 4 @ 300, and 4 @ 350. And the 4 pieces in each temperature group get baked for 30min, 60min, 90min, and 120min. He ends up recommending 350 for 60min.
The first time I attempted this i was trying to be cautious, so I only did 300 for 45min. That time I baked 2 sticks of purpleheart, 1 stick of padauk, and 1 stick of yellowheart. All darkened pretty good, so I thought i had selected a good temp and time. I was actually originally only going to do 30min, but I was monitoring the color and didn't see much change so I pushed it to 45min.
I used both the baked and unbaked pieces to make a color gradient serving board for my wife for our 5yr anniversary in November. This board has 11 unique species in it, by only the purpleheart, yellowheart, and padauk got baked (and it was only half of each that I baked). As you can see in the photos, the baked pieces are noticeably darker. Both the unbaked and baked pieces have retained all color to this day, but that board is covered in shellac, tabletop epoxy, and oil poly lol
Late winter of this year I baked purpleheart again. This time I was baking two 18" long pieces of 8/4 that were about 6" wide. Given the size, I decided to do 300 for 60min. These ones really didn't change much in shade. They were already very purple to start. I probably would have gone longer than 60min, but they began leaking out oils big time at the 60min mark (make sure you use junk cookie trays! ).
I used these pieces of 8/4 baked purpleheart to make a kitchen helper stool for my daughter. The finished pieces were treated with a penetrating varnish made of oil poly, raw tung, and mineral spirits. Then after curing I topcoated with satin acrylic poly. It's been about 3-4 months, and these still look exactly the same as the day I built them.
Photos 1-2: These show the results of my first bake at 300 for 45min. Unbaked pieces are next to baked pieces for comparison
Photo 3: this is some offcuts from the purpleheart in the first bake. I cut into them a few months later to show someone online how they remain purple throughout.
Photo 4: the serving board I made with the first batch of baked (and unbaked) wood
Photos 5-7: the kitchen helper I made my daughter using the second batch of baked purpleheart (300 for 60min).
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