Step 1: Make an oval
Go back and forth between hammering the washer on it’s edge and hammering it flat again until you have the shape you want.

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I really like the color you got. Also, thanks for posting the information you found relative to the colors (to brooklynlord's question).
I was really interested in the oxidising part...
Also, does oxidising it make it harder?
anvilfire.com Steel Temper Colors
Plain Carbon Steel Only - Hardness Rockwell C
English Color Text Fahrenheit Color Celsius SAE
1040 SAE
1050 SAE
1070 SAE
1095
Clear - As fully hardened 100° 38° 54 59 64 67
Pale yellow 2 420° 216° 51 55 59 63
Very pale yellow 1 430° 221°
Light yellow 1, 2, Straw 3 440° 227°
Pale straw-yellow 1, Straw 3 450° 232° 50 54
Straw-yellow 1, 2 460° 238° 62
Deep straw-yellow 1, 3 470° 243°
Dark yellow 1, Light orange 3 480° 249° 58
Yellow-brown 1, Orange 3 490° 254° 49 53 61
Brown-yellow 1, 3, Bronze 2 500° 260°
Spotted red-brown 1, Dark Brown 2 510° 266°
Brown with purple spots 1, 2 520° 271° 52 57
Light purple 1, 2, Purple 3 530° 277° 48 60
Full purple 1, Purple 2 540° 282°
Dark purple 1 550° 288°
Full blue 1, 3 560° 293° 47 51 56 59
Dark blue 1, Blue 2 570° 299°
Dark blue 2 590° 310° 58
Pale blue 3 46 50 55
Light blue 2 610° 321°
Greenish blue 2, Grey 3 630° 332° 45 54 57
Light blue 1 640° 338°
Steel grey 2 650° 343° 44 47 53 56
Steel grey 700° 371° 42 46 51 54
750° 400° 40 44 50 52
800° 427° 38 42 47 49
900° 482° 35 37 43 44
Lowest visible red 1000° 538° 30 32 36 39
English Colour Text Fahrenheit Colour Celsius SAE
1040 SAE
1050 SAE
1070 SAE
1095
Converted temps. rounded to nearest degree. Hardness +/- 1Rc @ +/-5°F of published data.
If I remember correctly, those steels would all be plain carbon steel with the last two numbers representing the carbon content. (1040 is 0.4% carbon, 1050 is 0.5% carbon, etc.) However, I'm not sure what treatment these numbers refer to, as how you cool the steel is at least as important as how hot you heat it.
That said, the treatment mrballeng is doing probably does affect the hardness, though I'm not sure which way.
You do beautiful work!