Do you literally have a supplier that is letting you call our the MC for a given order? That would be wild.
Our suppliers run material to industry standards. There is no asking for a half dozen boards to be taken to any given MC?
If your buying high quality material from a supplier that either saws their own material (and knows what they are doing), or sources from trusted sawyers, your source material should be of no concern.
We buy a good bit of plain sawn cherry from 4/4 on up from a spectacular supplier and have never had an issue. Any issues with regards to the material itself. Any issues will likely come from within the four walls of your shop.
The old adage of 6 to 8% MC really means 5.5% to 8.5% MC. This range is too much and for an interior table in a home is way too high. In most of the US, you would want 6.8% MC to 7.0% MC to avoid most warp. Note that ‘this is equivalent to 35% RH. Do you know the RH in the home or office, especially in the wintertime? Is it drier?
Note that any change in humidity will change the MC and this will result in size change including warp. A typical home runs from 30% RH to 50% RH (winter to summer) average which is 6.0% to 9.0% MC in the wood. This will result in over 1/3% size change in width, winter to summer. So do not fasten the top firmly to the frame, but allow for movement in width.
Also, make sure both top and bottom are finished the same so you get even moisture changes, top and bottom. This will really help prevent warp.
Always finish ALL sides of furniture with at least one good coat I use minimum of 2.....On the first round I always do the back (hidden) side first and immediately flip and do the other, this prevents usual MC changes from overnight drying in shops NOT having controlled MC/RH warping top. ALSO sticker up high for better airflow in drying. This makes MC changes equal on the wood and keeps good balance. ALSO correct joinery prevents future failures....tops need to float (actually any crossgrain woods)...NEVER glue wood cross grained.....VERY, VERY few exceptions!!!
Check out my website re wide slabs and furniture.....IT'S ALL about knowing the wood and good joinery!!!
Gene, Perhaps I am confused, but I am in the southern tier of Western N.Y. & the humidity in my house in the summer averages between 60 & 70% according to my "weather station". Winter is between 30 & 50%. Maybe RH is a different measurement than what my electronic unit is giving me. I only post this as I have made most of the furniture in our house & all the wood was air dried / shop dried to between 10 & 11.5% & I have not experienced any warping or stress cracks. Although, as has been suggested in this thread, all tops have been attached using figure 8 fasteners, not glued.
The outside RH in western N.Y. averages 70% RH in the summertime. It is below this during the day and higher in the nighttime, especially before sunrise when dew forms on the grass, etc. we can have the drier conditions in the daytime inside a house, but the high humidity in earlier morning will not happen in the house, as it is a bit warmer in the house and such high RH would be uncomfortable. It is therefore very unlikely that the inside of a home will be equal or more humid than the outside. Further, when humid outside air is heated such as in the winter, the RH quickly drops. Foggy air drops to under 30% RH with just 25 degrees of heat. So, the bottom line is that I suspect the weather station you have is not very accurate...your readings do not agree with the readings that others have taken.
Air dried followed by drying in the shop can achieve reasonable low moisture contents, even though the wood was not "kiln dried."
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