18th century slip-seat construction. How was it done?

I have made several reproduction chairs and stools over the years, all using slip seats that I made of 1/2" plywood, foam, and covering. But, how were these constructed in the 18th century? Were they done with webbing and horse hair? Were they solid and padded? Any idea what they felt like to sit in? While sitting on the pieces I made are not "painful", you can definitely feel that you are sitting on a solid board. Any ideas?
See the Woodwright's Shop video done at Colonial Williamsburg:

I have made 41 chairs and stools in our house. Most of the chairs have a slip seat while some have an upholstered seat (where the upholstery comes over the side rails). I make the slip seat frames about 2-1/2" wide and M&T the four corners. Some I had upholstered in horse hair to be authentic. However, the horse hair has no memory. After seating in them for years the seats are somewhat concave. The other seats were upholstered with foam and a thin layer of cotton. These are okay and keep their shape. Just remember to use a high grade of foam so it will not sag. And yes webbing was used on the slip seat frames.

Dennis Bork
Antiquity Period Designs, Ltd.
Here in Massachusetts are often made with through bridle joints. The trough tenons go on the side rails and the front and back rail gets the through mortises.

Ideal rail material is air dried Ash. 

Horse hair is the best choice and will outlive us all.  Second choice is rubberized horse hair, a hybrid treatment. The annual journal gives great information on this matter. The articles are written by Mike Mascelli.

You can purchase air filters which are blue and cut to fit which is rubberized hair. Lay two pieces down with a cotton cover over webbing. Make sure the slip seat has room to settle if constant pressure against all sides it can compromise joinery. Bevel the top edge of your mortise and tenoned slip seat even with the top edge. Webbing only goes on the top. You will need a webbing stretcher.    
When tacking webbing, first tack down with three tacks away from center then fold back over itself towards center and tack again with four tacks. Weave webbing across sides and front to back. Stretch webbing, tack with three tacks, fold back over and tack again with four tacks. Spit tacks  No staples. Staples might leave a bad taste in your mouth.
Figure at least an 1/8th" gap on all four sides for regular upholstery fabric or 1/4" on all four sides for tapestry or leather. Todays upholstery seems to over stuff.