The heat-absorbing filter should be used with the 23C for two reasons. It is indicated in the Beseler manual as recommended to protect any filter used in the lower filter drawer from heat. Without the filter, heat from the lamp will begin to destroy the dye in the filter, accelerating fading. See:
G. Heat-Absorbing Glass on page 4 of the manual for the Beseler 23CII.
http://www.jollinger.com/photo/cam-coll/manuals/enlargers/beseler/Beseler_23C_II.pdf
The heat-absorbing filter removes much of the heat from the lamp, so the negative receives far less heat. This will minimize—but not eliminate negative “popping”. In my 23CII, I have removed the sliding sheet-aluminum filter-holding drawer from the lower slot.
This allows me to use two heat-absorbing filters, one in the upper slot and one in the lower slot. All my filtering is done with the Ilford MG below-the-lens filter kit. This is my preferred contrast control for variable-contrast printing—even when using a dichroic-filtered color enlarger.
Even so, I use a glass negative carrier to ensure negative flatness during projection. It is the only way I know of to ensure uniform image focus, which is most noticeable when making large prints.
Each of these heat-absorbing glass filters (Beseler part 8042) holds back about 1/3 stop of light on its own.
The light intensity can be controlled with a lens-mounted neutral-density filter. Most enlarging lenses used on the Beseler 23C have a front thread of 40.5 mm x 0.5 mm, 43 mm x 0.75 mm. A few other sizes might be seen depending on the lens.
My 50mm f/2.8, 80 mm f/5.6, and 105 mm f/5.6 EL Nikkors use 40.5 mm x 0.5 mm front thread. I use an inexpensive, but well-made 40.5 mm to 52 mm step-up ring that allows me to use my Nikon camera-lens filters on these enlarger lenses when needed.
https://www.bhphotovideo.com/c/product/809750-REG/Sensei_sur40_552_40_5_52mm_Step_Up_Ring.html
You might already have a practical ND filter in the form of a polarizer. For example, the Hoya polarizer holds back about 5/3 stops for an exposure-time increase of 3.17X. Neutral density filters are commonly available in ND 0.3 (1 stop or 2X), ND 0.6 (2 stops or 4X), ND 0.9 (3 stops of 8X), Stronger ND filters are also available.
