Now that we have divided the platform into four parts, lets give them all a letter code: "A", "B", "C" and "D", as shown in fig. 1. I will discuss the following topics relating to each platform component.
1. It's purpose and/or function
2. "How to make them"
3. The attributes it should have
4. Trouble shooting... cause and effect of poorly made and or Improperly prepared platforms
Part A

The first we will look at is "A". This is the part commonly referred to as the "bevel". The purpose of the "bevel" is that it serves as the surface that is actually struck to produce the flake. How do we go about making the bevel? The most accurate way is to use a SHARP pressure flaker. You can use a billet to produce this on an early stage perform or spall. It is highly recommended that you use a pressure flaker to make this part. What attributes should part "A" have? This part should have a bevel some where near 45 degrees. This angle can be changed by making another "pass" or modifying angle of pressure. The bevel should be smooth. What I mean by this is that it should not contain irregular bumps, humps and micro ridges. It should be just as if you used a router on a piece of wood.
Part B

Part "B" gives support to the strike. It is actually made from part "A". There are many ways to make this. The basic idea is that you're actually removing extremely tiny chips off the bottom or underside of the "bevel". (This is the same side the thinning flake will be removed from.) Remember, your not really abrading the edge so much as shaping it. Here's a couple ways of doing this. The first way is to use a course abrader. Just rake the edge downward gently and repeating this process just long enough to feel less resistance as the abrader is raked downward. You can also rake off these "micro" flakes with the edge of your pressure flaker or use a copper bar to do the same thing. Keep in mind this is a very important step! If you rake it too hard or use or use excessive force it will be too strong and will greatly stress the stone upon striking it. Rake thick performs harder than thin ones. If "B" is not raked enough it will cause the platform to crush or cause a step fracture very close to the edge. Too much and you will break it! So don't over do it.
Part C

Moving onto part "C". This part is also made from "A". It is the polished area that your billet actually strikes. It is better described as polished but commonly referred to as abraded. Polishing sounds so much more precise and civilized. To prepare this part properly one must first have created "A" and "B" flawlessly! You simply grind up and down the platform edge. What I mean by this is your grinding from base to tip. Another description of this is if you're holding the preform flat, the grinding motion is horizontal NOT vertical. A vertical motion will destroy the platform. You want to use course abraders for preforms thicker than 5 to 1 width to thickness and a medium abrader for thinner bi-faces. Be cautious not to over grind, this will also cause splits or breakage. Keep in mind... the better you make your platforms... the less grinding they will need!
Part D

Finally part "D". This is what I like to refer to as the "road" the flake will travel down. This must be closely looked at before you decide to remove any material for the purpose of platform construction. If the surface area of part "D" is irregular, then it must be corrected before an attempt at flake removal is made. Simply put, don't waist the time and circumference of your bi-face trying to chip off an area with a stack or concavity. Just work on either side of it. Build platforms to target areas with good convexities. Stay away from concavities. You can modify the surface of your bi-face by pressure flaking if necessary. You must be careful not to cause "micro" steps with your billet or Ishi stick. It will just be more trash for your thinning flake to contend with.
Just remember to take your time and analyze. Be safe and have fun!

- Mark Bracken
Flintknappingtools.com