RH and what you need to know
Posted: Thu Sep 12, 2013 1:02 pm
Many of you may be just starting out in making guitars . Often the last thing you may think about is the shops RH ( Relative Humidity ) . This hidden environment can help or hurt you and often won't raise its ugly head until it is too late.
You can measure the RH in your shop with a hygromemter. I have yet to find an accurate digital one. I use a good old fashion analog dial unit.
So what does or what doesn't RH do? Well this is a way to measure the amount of moisture in the air. Too much moisture can cause the wood to swell across the grain and too little can cause it to shrink. All movement is across the wood grain.
So with this knowlege you can now start to understand the importance of knowing and controlling this.
For most conditions 45 to 55% is acceptable. What I will do, as most issues are with extreme conditions is to use this in my favor. A top will contract and once the braces are applied you have locked up the plate. As the wood dries out it will contract. This will make the top sink, and if you have a dome in your top you will see this lower to the point it will sink so badly you can see a convex top.
This is where the top can be so stressed that it will split. This is not a good thing. To help avoid this you have a few options. One is to never let the guitar out of the house and maintain the proper RH , the other is to use the information and apply this to the building process.
Glues used to attach the braces has a water content. When you brace a top you are adding water , this can cause the top to swell slighty but you may not perceive this movement. I liken it to adding about 10% more RH than the natural environment has.
I can lower my RH in the shop by turning a knob and pulling the excess moisture out of the air. When I am planning to bracing plate I will set the RH to 35 to 35%. The water in the glue will have little effect down the road.
If you don't have this option you can do what I used to before I had the control. I used an iron and literally ironed the plates. I did avoid working the joint . You will see in a few mintues of heat ( NO STEAM) just dry heat the moisture wicking out of the end grain.
I would do this for about 5 minutes. You can also "bake the top" but I find this method if not careful can stress the seam joint.
So after a few minutes of heating I would then brace the top. I would also let this in the go-bar deck on the radius dish for at least 24hr. This will help your top in RH stress to the negative. Come winter this may save you from a seal separation.
I would still use a case humdifier once the RHdrops below 45% and keep an eye on the instrument for any signs of RH stress.
These signs are
Sharp fret ends
Grain courdoroy
and top dome sinking.
Once you see these signs be sure you use the humdifier.
Hope you find this information helpful
john hall
Blues Creek Guitars.
You can measure the RH in your shop with a hygromemter. I have yet to find an accurate digital one. I use a good old fashion analog dial unit.
So what does or what doesn't RH do? Well this is a way to measure the amount of moisture in the air. Too much moisture can cause the wood to swell across the grain and too little can cause it to shrink. All movement is across the wood grain.
So with this knowlege you can now start to understand the importance of knowing and controlling this.
For most conditions 45 to 55% is acceptable. What I will do, as most issues are with extreme conditions is to use this in my favor. A top will contract and once the braces are applied you have locked up the plate. As the wood dries out it will contract. This will make the top sink, and if you have a dome in your top you will see this lower to the point it will sink so badly you can see a convex top.
This is where the top can be so stressed that it will split. This is not a good thing. To help avoid this you have a few options. One is to never let the guitar out of the house and maintain the proper RH , the other is to use the information and apply this to the building process.
Glues used to attach the braces has a water content. When you brace a top you are adding water , this can cause the top to swell slighty but you may not perceive this movement. I liken it to adding about 10% more RH than the natural environment has.
I can lower my RH in the shop by turning a knob and pulling the excess moisture out of the air. When I am planning to bracing plate I will set the RH to 35 to 35%. The water in the glue will have little effect down the road.
If you don't have this option you can do what I used to before I had the control. I used an iron and literally ironed the plates. I did avoid working the joint . You will see in a few mintues of heat ( NO STEAM) just dry heat the moisture wicking out of the end grain.
I would do this for about 5 minutes. You can also "bake the top" but I find this method if not careful can stress the seam joint.
So after a few minutes of heating I would then brace the top. I would also let this in the go-bar deck on the radius dish for at least 24hr. This will help your top in RH stress to the negative. Come winter this may save you from a seal separation.
I would still use a case humdifier once the RHdrops below 45% and keep an eye on the instrument for any signs of RH stress.
These signs are
Sharp fret ends
Grain courdoroy
and top dome sinking.
Once you see these signs be sure you use the humdifier.
Hope you find this information helpful
john hall
Blues Creek Guitars.