Critical Angle

[mehoose]

Yes I know I need to get a book.
Lets look at Quartz.
If they say critical angle for pav is 42 and Crn is 41, then which way is the wrong way for cutting above/below?
Could you please respond with measurements so I get it.
 
Merryn.

[moonshadow_dancer]

Lets see, CA for quartz pavilion is 40.49, I prefer at least 42, the crown I am not sure of as I try to cut star facet of A SRB at 22, and also remember the table facet is 0, whatever stone you are cuttin you have to be above the critical angle on the pav, so your lowest facet for example if the CA is 40.49 then in theory all you facets must be above that like 41 and up to avoid being a fish eye as the critical angle is the lowest possible you can go, a degree or 2 above to me is better, but never lower.

Gee hope that made sense and I hope it helps. 

Take Care
Deb 

[mehoose]

Yes, thank you.

[moonshadow_dancer]

I am trying to go back into my memory bank which is hard with a few vino's under my belt but I believe Facet Cutters Handbook - BY Edward Soukup
http://www.aussiesapphire.com.au/index.php?main_page=product_info&cPath=62&products_id=1130
Has a table of different gem material and al the main angles etc needed.

I see it is sold out at the moment but it is a good book for the price and very handy.

[Pete D]

For PAVILION ANGLES with an RI of 1.6 & under you can use 43  degrees for mains ,for RI above 1.6 use 42  degrees.
For  CROWN start with about 38 degrees will give good results, for gems with an RI or 1.7 to 1.8 [garnet , sapphire] you could reduce angel as low as 25 degrees to return more light to the eye instead of being reflected back into the stone [extinction] . Lower crown mains dose increase face up brilliance.  Dispersion cane be increased by cutting crown mains at around 45 degrees,only some stones will show dispersion [the splitting of light into it separate colours i.e. red flash or fire]
 CZ is a good example.  I hope this is helpful.         

[moonshadow_dancer]

For PAVILION ANGLES with an RI of 1.6 & under you can use 43  degrees for mains ,for RI above 1.6 use 42  degrees.
For  CROWN start with about 38 degrees will give good results, for gems with an RI or 1.7 to 1.8 [garnet , sapphire] you could reduce angel as low as 25 degrees to return more light to the eye instead of being reflected back into the stone [extinction] . Lower crown mains dose increase face up brilliance.  Dispersion cane be increased by cutting crown mains at around 45 degrees,only some stones will show dispersion [the splitting of light into it separate colours i.e. red flash or fire]
 CZ is a good example.  I hope this is helpful.         

Thanks Pete,

That is a very well descriptive way of understanding Critical Angles.

I look forward to reading more of your posts and shared knowledge.

Take Care
Deb 

[daleywaters]

Hi all, just a link that may be of use with CA's and the like.

http://www.usfacetersguild.org/gemstone_properties.shtml

Cheers,
Dale

[daleywaters]

And this too.  Hope it helps.

http://www.facetorsguild.org.au/gem_angles.htm

[gussyy]

Hi, When it gives you the critical angle how do you know which angle it is and whether it is for the crown or pavillion. Like sapphire critical angle is 34deg but which facet is it referring to.

                                                    Cheers.

[gammyknee]

Hi Gussyy,
I am learning this myself.
A book I am reading gives this description.
The CA, a mathematical function of the Refractive Index, represents the last angle below which light rays will no longer be reflected from the surface of the material. So it does not actually equal a given facet angle that you are looking for.

To give a example to help understand this - when skipping stones over smooth water at an angle, the stone (light) will 'reflect' off the surface. If you steepen the angle to the water it will reach a CA where the stone (light) will not reflect.
The angle is measured from the perpendicular, straight down to what is called 'normal to the plane'.

The AFG table that daleywaters has given you the link for, will give you the CA on the far right for some given material and under the Pavillion heading it will give a recommended angle for the mains and the girdle (break) facets. These angles have been calculated to provide the best return of light given the CA. If you are looking for other material recommended angles let us know - I think I have list somewhere - I seem to be doing more reading than cutting at present.

Hope this helps. Maybe someone could confirm or improve on this explanation.

[gammyknee]

Actually Deb has hit it on the head in the 2nd post here and also Pete D.

Michael

[moonshadow_dancer]

Hi Gussyy,

Lets see, if you are cutting a sapphire in a Standard round brilliant and its CA is 34.62°, then what I call your Girdle facets being 3,9,15,etc I usually do at 40^o for sapphire and the mains at no lower then 37 ^o Quartz I won't go below 45 for the girdles, so the ones you need to be concerned about are the last row of facets that make your culet. In saying this I like to go even higher with sapphires in light material, these are the angles I use for darker material. For the crown on a sapphire I use 40 for the girdles, 35 for the mains and 22 for the stars.

I have attached a sRB pattern I use for sapphires and zircon.

Hope this helps.

Take Care
Deb