Aging wood instantly with potash
A chemical stain that works magic on tannin-rich woods
How can you skip dyes and stains but still achieve the rich hues and grain clarity reminiscent of a centuries-old patina? If your project is made of tannin-rich woods such as mahogany, cherry, walnut, or oak, then potassium dichromate—or potash—is a great choice. As a bonus, potash won’t darken woods without tannins, so inlaid wood such as holly stringing will stay nice and white.
What is potash?
Potash is a commercially manufactured granular powder with the same chemical makeup (K2Cr2O7) as the naturally occurring orange crystal lopezite. Mixed with warm water, potash forms a chemical stain (the name potash can also refer to a range of other compounds containing potassium). As an oxidizing agent, potassium dichromate is used in various industrial applications such as leather tanning and black-and-white photography. When applied to certain woods, most notably mahogany, cherry, and walnut, potash reacts with the tannins in those woods to accelerate the natural aging process and produce an aged depth of color and shimmering grain pop that is unparalleled.
Although steeped in furniture finishing history, potash is overshadowed today by more conventional and readily available products. However, with some online digging, you can purchase 1 lb. of potassium dichromate for less than $20. I get mine from pyrochemsource.com.
The chemical makeup of potassium dichromate is considered toxic. This has diminished its potential as a favorable, modern-day finishing agent. However, with common sense precautions, potash is a safe finish worth including in your arsenal of finishing options.
When handling potassium dichromate—either in its pure granular or water-mixed form—wear rubber gloves, eye protection, a dust mask, and clothing that covers your arms and legs. Mix the solution in a ventilated area and, to avoid splashing, add the powder to the water instead of the other way around. Safely store any unused solution in a clearly marked and covered plastic or glass container or use it up by spreading it on scrapwood. Either in its granular form or in solution, potash will last indefinitely. Once dry, it’s no longer considered hazardous.
To get ready for the potash application, scrape the surface of your workpiece until it is flat and even. On open-grain woods, raise the grain by applying a generous amount of water to the surface with a clean, wet rag. After the surface has dried, sand away the fuzz with 220-grit paper. Because potash is a water-based solution, repeat this process to minimize post-solution sanding.
If you are using an open-grain wood and want a closed-grain finish, you’ll need to fill the grain first, then prep for the potash. As a side note, the scraping and surface preparation steps described above don’t apply to crotch veneers because of their inherent fragility and variations in grain structure. However, these veneers should be carefully sanded to 220-grit and wiped clean in preparation for potash.
What about blotch-prone woods?
Cherry is known for its blotchiness if not properly prepared for finishing. You can minimize or prevent blotching by doing a blotch test with mineral spirits and then sealing the wood. Start by scraping the surface flat and even. Look for blotch-prone areas by wiping the cherry with a clean rag soaked in mineral spirits. Treat blotching with a washcoat of dewaxed shellac, followed by sanding with 220- to 300-grit paper.
Apply the potash
When applied, the potash solution will be bright yellow in color and will not immediately change the color of the wood. Over the course of the next several hours, the wood will darken to its final color. Let it set at least 10 hours to complete the chemical process before applying a topcoat of your choice.
Dan Strout makes furniture in Milton, Mass.
More on FineWoodworking.com:
- Easy Finish Ages a Classic Cherry Piece – Simple steps to mute color of new cherry wood and create a durable finish
- Anyone for Tea? – Low-tech dye adds subtle age and won’t blotch
- Dry-Brushing Wood Stains – Widen your range of color possibilities using stains and tints